burn_dispatch/

backend.rs

use alloc::format;
use alloc::string::String;
use alloc::vec::Vec;

#[cfg(any(
    feature = "cpu",
    feature = "ndarray",
    feature = "flex",
    default_backend
))]
use alloc::vec;

#[cfg(feature = "autodiff")]
use burn_backend::distributed::{DistributedParamId, DistributedParams};
use burn_backend::{AutodiffBackend, Backend, BackendGraph, BackendTypes, DType, ExecutionError};

/// A captured graph from one of the dispatched backends (see
/// [`BackendTypes::GraphPrimitive`]).
///
/// Like [`DispatchTensorKind`], one variant per enabled backend: the graph is
/// captured by, and can only replay on, the backend it was recorded on.
#[derive(Debug, Clone)]
pub enum DispatchGraph {
    /// A graph captured on the [CPU backend](Cpu).
    #[cfg(feature = "cpu")]
    Cpu(BackendGraph<Cpu>),

    /// A graph captured on the [CUDA backend](Cuda).
    #[cfg(feature = "cuda")]
    Cuda(BackendGraph<Cuda>),

    /// A graph captured on the [Metal backend](Metal).
    #[cfg(feature = "metal")]
    Metal(BackendGraph<Metal>),

    /// A graph captured on the [ROCm backend](Rocm).
    #[cfg(feature = "rocm")]
    Rocm(BackendGraph<Rocm>),

    /// A graph captured on the [Vulkan backend](Vulkan).
    #[cfg(feature = "vulkan")]
    Vulkan(BackendGraph<Vulkan>),

    /// A graph captured on the [Wgpu backend](Wgpu).
    #[cfg(feature = "wgpu")]
    Wgpu(BackendGraph<Wgpu>),

    /// A graph captured on the [WebGPU backend](WebGpu).
    #[cfg(feature = "webgpu")]
    WebGpu(BackendGraph<WebGpu>),

    /// A graph captured on the [Flex backend](Flex).
    #[cfg(any(feature = "flex", default_backend))]
    Flex(BackendGraph<Flex>),

    /// A graph captured on the [NdArray backend](NdArray).
    #[cfg(feature = "ndarray")]
    NdArray(BackendGraph<NdArray>),

    /// A graph captured on the [LibTorch backend](LibTorch).
    #[cfg(feature = "tch")]
    LibTorch(BackendGraph<LibTorch>),

    /// A graph captured on the [Remote backend](Remote).
    #[cfg(feature = "remote")]
    Remote(BackendGraph<Remote>),
}

/// The error returned when a graph operation cannot be dispatched.
fn graph_dispatch_err(reason: alloc::string::String) -> ExecutionError {
    ExecutionError::WithContext { reason }
}

/// Match arm generator for [`Backend::graph_stop_capture`] on [`Dispatch`]:
/// each backend's captured graph is wrapped in its [`DispatchGraph`] variant.
macro_rules! graph_stop_capture_arms {
    ($device:expr; $([$Backend:ident, $cfg:meta]),*) => {
        match $device {
            $(
                #[cfg($cfg)]
                $crate::DispatchDevice::$Backend(device) => {
                    <$crate::backends::$Backend as Backend>::graph_stop_capture(device)
                        .map(DispatchGraph::$Backend)
                }
            )*
            #[allow(unreachable_patterns)]
            other => Err(graph_dispatch_err(format!(
                "Graph capture is not supported for device {other:?}"
            ))),
        }
    };
}

/// Match arm generator for [`Backend::graph_replay`] on [`Dispatch`]: the graph
/// variant must match the device's backend, since a graph only replays on the
/// backend that captured it.
macro_rules! graph_replay_arms {
    ($device:expr, $graph:expr; $([$Backend:ident, $cfg:meta]),*) => {
        match ($device, $graph) {
            $(
                #[cfg($cfg)]
                ($crate::DispatchDevice::$Backend(device), DispatchGraph::$Backend(graph)) => {
                    // Safety: forwarded verbatim from `Dispatch::graph_replay`'s
                    // own contract.
                    unsafe {
                        <$crate::backends::$Backend as Backend>::graph_replay(device, graph)
                    }
                }
            )*
            #[allow(unreachable_patterns)]
            (device, _) => Err(graph_dispatch_err(format!(
                "The graph was not captured on the backend of device {device:?}"
            ))),
        }
    };
}

#[cfg(feature = "autodiff")]
use alloc::boxed::Box;
#[cfg(feature = "autodiff")]
use burn_autodiff::grads::Gradients;

#[allow(unused)]
use crate::DispatchDeviceId;
#[allow(unused)]
use crate::DispatchTensorKind;
use crate::backends::*;
use crate::{DispatchDevice, DispatchTensor};

/// The main execution backend in Burn.
///
/// [`Dispatch`] acts as a global backend that can manage multiple underlying
/// backends (e.g., `Cpu`, `Cuda`, `Wgpu`, `Metal`, etc.).
/// It is responsible for:
/// - Dispatching tensor operations to the appropriate backend.
/// - Managing cross-backend tensor transfers.
///
/// Essentially, [`Dispatch`] is the single entry point for executing tensor operations
/// in a backend-agnostic way. It allows Burn to provide a unified, global backend
/// for users while still leveraging multiple specialized backends under the hood.
///
/// # Example
///
/// ```ignore
/// use burn::Dispatch;
/// use burn::DispatchDevice;
///
/// // Select the device to execute operations on
/// let device = DispatchDevice::Cuda(Default::default());
///
/// // Create a tensor using the global backend
/// let t = Tensor::<Dispatch, 2>::zeros([128, 128], &device);
/// ```
#[derive(Debug, Default, Clone)]
pub struct Dispatch;

impl BackendTypes for Dispatch {
    type Device = DispatchDevice;

    type FloatTensorPrimitive = DispatchTensor;
    type IntTensorPrimitive = DispatchTensor;
    type BoolTensorPrimitive = DispatchTensor;
    type QuantizedTensorPrimitive = DispatchTensor;

    type GraphPrimitive = DispatchGraph;
}

impl Backend for Dispatch {
    fn name(device: &Self::Device) -> String {
        let inner = dispatch_device!(device, |device| B::name(device));
        format!("dispatch<{inner}>")
    }

    fn seed(device: &Self::Device, seed: u64) {
        dispatch_device!(device, |device| B::seed(device, seed))
    }

    fn sync(device: &Self::Device) -> Result<(), ExecutionError> {
        dispatch_device!(device, |device| B::sync(device))
    }

    fn graph_prepare(device: &Self::Device) -> Result<(), ExecutionError> {
        dispatch_device!(device, |device| B::graph_prepare(device))
    }

    fn graph_start_capture(device: &Self::Device) -> Result<(), ExecutionError> {
        dispatch_device!(device, |device| B::graph_start_capture(device))
    }

    fn graph_stop_capture(device: &Self::Device) -> Result<DispatchGraph, ExecutionError> {
        backend_list!(graph_stop_capture_arms, device)
    }

    unsafe fn graph_replay(
        device: &Self::Device,
        graph: &DispatchGraph,
    ) -> Result<(), ExecutionError> {
        backend_list!(graph_replay_arms, device, graph)
    }

    fn dtype_usage(device: &Self::Device, dtype: DType) -> burn_backend::DTypeUsageSet {
        dispatch_device!(device, |device| B::dtype_usage(device, dtype))
    }

    fn ad_enabled(device: &Self::Device) -> bool {
        match device {
            #[cfg(feature = "autodiff")]
            DispatchDevice::Autodiff(_) => true,
            _ => false,
        }
    }

    fn device_count(type_id: u16) -> usize {
        let (dispatch_id, backend_type_id) = DispatchDevice::decode_type_id(type_id);
        match dispatch_id {
            #[cfg(feature = "cpu")]
            DispatchDeviceId::Cpu => Cpu::device_count(backend_type_id),
            #[cfg(feature = "cuda")]
            DispatchDeviceId::Cuda => Cuda::device_count(backend_type_id),
            #[cfg(feature = "metal")]
            DispatchDeviceId::Metal => Metal::device_count(backend_type_id),
            #[cfg(feature = "rocm")]
            DispatchDeviceId::Rocm => Rocm::device_count(backend_type_id),
            #[cfg(feature = "vulkan")]
            DispatchDeviceId::Vulkan => Vulkan::device_count(backend_type_id),
            #[cfg(feature = "wgpu")]
            DispatchDeviceId::Wgpu => Wgpu::device_count(backend_type_id),
            #[cfg(feature = "webgpu")]
            DispatchDeviceId::WebGpu => WebGpu::device_count(backend_type_id),
            #[cfg(any(feature = "flex", default_backend))]
            DispatchDeviceId::Flex => Flex::device_count(backend_type_id),
            #[cfg(feature = "ndarray")]
            DispatchDeviceId::NdArray => NdArray::device_count(backend_type_id),
            #[cfg(feature = "tch")]
            DispatchDeviceId::LibTorch => LibTorch::device_count(backend_type_id),
            #[cfg(feature = "remote")]
            DispatchDeviceId::Remote => Remote::device_count(backend_type_id),
            _ => unreachable!("No backend feature enabled."),
        }
    }

    fn memory_persistent_allocations<
        Output: Send,
        Input: Send,
        Func: Fn(Input) -> Output + Send,
    >(
        device: &Self::Device,
        input: Input,
        func: Func,
    ) -> Output {
        dispatch_device!(device, |device| B::memory_persistent_allocations(
            device, input, func
        ))
    }

    fn memory_cleanup(device: &Self::Device) {
        dispatch_device!(device, |device| B::memory_cleanup(device))
    }

    fn staging<'a, Iter>(data: Iter, device: &Self::Device)
    where
        Iter: Iterator<Item = &'a mut burn_backend::TensorData>,
    {
        dispatch_device!(device, |device| B::staging(data, device))
    }

    fn supports_dtype(device: &Self::Device, dtype: DType) -> bool {
        dispatch_device!(device, |device| B::supports_dtype(device, dtype))
    }

    fn flush(device: &Self::Device) {
        dispatch_device!(device, |device| B::flush(device))
    }
}

#[cfg(feature = "autodiff")]
impl AutodiffBackend for Dispatch {
    type InnerBackend = Dispatch;

    type Gradients = Gradients;

    fn backward(tensor: DispatchTensor) -> Self::Gradients {
        let DispatchTensor { kind, .. } = tensor;

        match kind {
            DispatchTensorKind::Autodiff(tensor) => match *tensor {
                #[cfg(feature = "cpu")]
                DispatchTensorKind::Cpu(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "metal")]
                DispatchTensorKind::Metal(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "rocm")]
                DispatchTensorKind::Rocm(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "vulkan")]
                DispatchTensorKind::Vulkan(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "wgpu")]
                DispatchTensorKind::Wgpu(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "webgpu")]
                DispatchTensorKind::WebGpu(tensor) => tensor.autodiff().backward(),
                #[cfg(any(feature = "flex", default_backend))]
                DispatchTensorKind::Flex(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "ndarray")]
                DispatchTensorKind::NdArray(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "tch")]
                DispatchTensorKind::LibTorch(tensor) => tensor.autodiff().backward(),
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => tensor.autodiff().backward(),
                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
            },
            _ => panic!("Requires autodiff tensor."),
        }
    }

    fn grad(tensor: &DispatchTensor, grads: &Self::Gradients) -> Option<DispatchTensor> {
        let DispatchTensor {
            kind,
            checkpointing,
        } = tensor;
        let grad: Option<DispatchTensorKind> = match &kind {
            DispatchTensorKind::Autodiff(inner_kind) => match &**inner_kind {
                #[cfg(feature = "cpu")]
                DispatchTensorKind::Cpu(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Cpu(crate::BackendTensor::Float(t))),
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Cuda(crate::BackendTensor::Float(t))),
                #[cfg(feature = "metal")]
                DispatchTensorKind::Metal(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Metal(crate::BackendTensor::Float(t))),
                #[cfg(feature = "rocm")]
                DispatchTensorKind::Rocm(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Rocm(crate::BackendTensor::Float(t))),
                #[cfg(feature = "vulkan")]
                DispatchTensorKind::Vulkan(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Vulkan(crate::BackendTensor::Float(t))),
                #[cfg(feature = "wgpu")]
                DispatchTensorKind::Wgpu(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Wgpu(crate::BackendTensor::Float(t))),
                #[cfg(feature = "webgpu")]
                DispatchTensorKind::WebGpu(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::WebGpu(crate::BackendTensor::Float(t))),
                #[cfg(any(feature = "flex", default_backend))]
                DispatchTensorKind::Flex(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Flex(crate::BackendTensor::Float(t))),
                #[cfg(feature = "ndarray")]
                DispatchTensorKind::NdArray(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::NdArray(crate::BackendTensor::Float(t))),
                #[cfg(feature = "tch")]
                DispatchTensorKind::LibTorch(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::LibTorch(crate::BackendTensor::Float(t))),
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => tensor
                    .as_autodiff()
                    .grad(grads)
                    .map(|t| DispatchTensorKind::Remote(crate::BackendTensor::Float(t))),
                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
            },
            _ => panic!("Requires autodiff tensor."),
        };
        grad.map(|kind| DispatchTensor {
            kind,
            checkpointing: *checkpointing,
        })
    }

    fn grad_remove(tensor: &DispatchTensor, grads: &mut Self::Gradients) -> Option<DispatchTensor> {
        let DispatchTensor {
            kind,
            checkpointing,
        } = tensor;
        let grad: Option<DispatchTensorKind> = match &kind {
            DispatchTensorKind::Autodiff(inner_kind) => match &**inner_kind {
                #[cfg(feature = "cpu")]
                DispatchTensorKind::Cpu(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Cpu(crate::BackendTensor::Float(t))),
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Cuda(crate::BackendTensor::Float(t))),
                #[cfg(feature = "metal")]
                DispatchTensorKind::Metal(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Metal(crate::BackendTensor::Float(t))),
                #[cfg(feature = "rocm")]
                DispatchTensorKind::Rocm(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Rocm(crate::BackendTensor::Float(t))),
                #[cfg(feature = "vulkan")]
                DispatchTensorKind::Vulkan(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Vulkan(crate::BackendTensor::Float(t))),
                #[cfg(feature = "wgpu")]
                DispatchTensorKind::Wgpu(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Wgpu(crate::BackendTensor::Float(t))),
                #[cfg(feature = "webgpu")]
                DispatchTensorKind::WebGpu(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::WebGpu(crate::BackendTensor::Float(t))),
                #[cfg(any(feature = "flex", default_backend))]
                DispatchTensorKind::Flex(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Flex(crate::BackendTensor::Float(t))),
                #[cfg(feature = "ndarray")]
                DispatchTensorKind::NdArray(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::NdArray(crate::BackendTensor::Float(t))),
                #[cfg(feature = "tch")]
                DispatchTensorKind::LibTorch(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::LibTorch(crate::BackendTensor::Float(t))),
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => tensor
                    .as_autodiff()
                    .grad_remove(grads)
                    .map(|t| DispatchTensorKind::Remote(crate::BackendTensor::Float(t))),
                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
            },
            _ => panic!("Requires autodiff tensor."),
        };
        grad.map(|kind| DispatchTensor {
            kind,
            checkpointing: *checkpointing,
        })
    }

    fn grad_replace(tensor: &DispatchTensor, grads: &mut Self::Gradients, grad: DispatchTensor) {
        let DispatchTensor {
            kind,
            checkpointing,
        } = tensor;
        let DispatchTensor {
            kind: grad,
            checkpointing: grad_ckp,
        } = grad;
        debug_assert_eq!(checkpointing, &grad_ckp);

        match &kind {
            DispatchTensorKind::Autodiff(inner_kind) => match (&**inner_kind, grad) {
                #[cfg(feature = "cpu")]
                (DispatchTensorKind::Cpu(tensor), DispatchTensorKind::Cpu(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "cuda")]
                (DispatchTensorKind::Cuda(tensor), DispatchTensorKind::Cuda(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "metal")]
                (DispatchTensorKind::Metal(tensor), DispatchTensorKind::Metal(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "rocm")]
                (DispatchTensorKind::Rocm(tensor), DispatchTensorKind::Rocm(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "vulkan")]
                (DispatchTensorKind::Vulkan(tensor), DispatchTensorKind::Vulkan(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "wgpu")]
                (DispatchTensorKind::Wgpu(tensor), DispatchTensorKind::Wgpu(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "webgpu")]
                (DispatchTensorKind::WebGpu(tensor), DispatchTensorKind::WebGpu(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(any(feature = "flex", default_backend))]
                (DispatchTensorKind::Flex(tensor), DispatchTensorKind::Flex(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "ndarray")]
                (DispatchTensorKind::NdArray(tensor), DispatchTensorKind::NdArray(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                #[cfg(feature = "remote")]
                (DispatchTensorKind::Remote(tensor), DispatchTensorKind::Remote(grad)) => {
                    tensor.as_autodiff().grad_replace(grads, grad.float())
                }
                (DispatchTensorKind::Autodiff(_), _) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
                // TODO: distributed message?
                (t, g) => panic!(
                    "The provided tensors are not on the same backend. Got backends {t:?} and {g:?}."
                ),
            },
            _ => panic!("Requires autodiff tensor."),
        }
    }

    fn inner(tensor: DispatchTensor) -> DispatchTensor {
        let DispatchTensor {
            kind,
            checkpointing: _,
        } = tensor;

        let kind = match kind {
            DispatchTensorKind::Autodiff(inner_kind) => match *inner_kind {
                #[cfg(feature = "cpu")]
                DispatchTensorKind::Cpu(tensor) => DispatchTensorKind::Cpu(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => DispatchTensorKind::Cuda(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "metal")]
                DispatchTensorKind::Metal(tensor) => DispatchTensorKind::Metal(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "rocm")]
                DispatchTensorKind::Rocm(tensor) => DispatchTensorKind::Rocm(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "vulkan")]
                DispatchTensorKind::Vulkan(tensor) => DispatchTensorKind::Vulkan(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "wgpu")]
                DispatchTensorKind::Wgpu(tensor) => DispatchTensorKind::Wgpu(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "webgpu")]
                DispatchTensorKind::WebGpu(tensor) => DispatchTensorKind::WebGpu(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(any(feature = "flex", default_backend))]
                DispatchTensorKind::Flex(tensor) => DispatchTensorKind::Flex(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "ndarray")]
                DispatchTensorKind::NdArray(tensor) => DispatchTensorKind::NdArray(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "tch")]
                DispatchTensorKind::LibTorch(tensor) => DispatchTensorKind::LibTorch(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => DispatchTensorKind::Remote(
                    crate::BackendTensor::Float(tensor.autodiff().primitive),
                ),
                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
            },
            _ => panic!("Requires autodiff tensor."),
        };
        DispatchTensor {
            kind,
            checkpointing: None,
        }
    }

    fn int_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    fn bool_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    fn q_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    fn from_inner(tensor: DispatchTensor) -> DispatchTensor {
        let DispatchTensor {
            kind,
            checkpointing,
        } = tensor;

        let kind = match kind {
            #[cfg(feature = "cpu")]
            DispatchTensorKind::Cpu(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Cpu(
                    crate::BackendTensor::Autodiff(Autodiff::<Cpu>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "cuda")]
            DispatchTensorKind::Cuda(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Cuda(
                    crate::BackendTensor::Autodiff(Autodiff::<Cuda>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "metal")]
            DispatchTensorKind::Metal(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Metal(
                    crate::BackendTensor::Autodiff(Autodiff::<Metal>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "rocm")]
            DispatchTensorKind::Rocm(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Rocm(
                    crate::BackendTensor::Autodiff(Autodiff::<Rocm>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "vulkan")]
            DispatchTensorKind::Vulkan(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Vulkan(
                    crate::BackendTensor::Autodiff(Autodiff::<Vulkan>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "wgpu")]
            DispatchTensorKind::Wgpu(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Wgpu(
                    crate::BackendTensor::Autodiff(Autodiff::<Wgpu>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "webgpu")]
            DispatchTensorKind::WebGpu(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::WebGpu(
                    crate::BackendTensor::Autodiff(Autodiff::<WebGpu>::from_inner(tensor.float())),
                )))
            }
            #[cfg(any(feature = "flex", default_backend))]
            DispatchTensorKind::Flex(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Flex(
                    crate::BackendTensor::Autodiff(Autodiff::<Flex>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "ndarray")]
            DispatchTensorKind::NdArray(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::NdArray(
                    crate::BackendTensor::Autodiff(Autodiff::<NdArray>::from_inner(tensor.float())),
                )))
            }
            #[cfg(feature = "tch")]
            DispatchTensorKind::LibTorch(tensor) => DispatchTensorKind::Autodiff(Box::new(
                DispatchTensorKind::LibTorch(crate::BackendTensor::Autodiff(
                    Autodiff::<LibTorch>::from_inner(tensor.float()),
                )),
            )),
            #[cfg(feature = "remote")]
            DispatchTensorKind::Remote(tensor) => {
                DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Remote(
                    crate::BackendTensor::Autodiff(Autodiff::<Remote>::from_inner(tensor.float())),
                )))
            }
            DispatchTensorKind::Autodiff(_) => {
                panic!("Autodiff should not wrap an autodiff tensor.")
            }
        };

        // TODO: should use C::STRATEGY
        let checkpointing = if let Some(strategy) = checkpointing {
            Some(strategy)
        } else {
            Some(crate::CheckpointingStrategy::None)
        };
        DispatchTensor {
            kind,
            checkpointing,
        }
    }

    fn int_from_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    fn bool_from_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    fn q_from_inner(tensor: DispatchTensor) -> DispatchTensor {
        tensor
    }

    // Only the collective-capable backends (Cuda/Remote) carry distributed params; in builds
    // without them the match arms cfg out, leaving the bindings unused and the tail unreachable.
    #[allow(unused_variables, unreachable_code)]
    fn set_distributed_params(
        tensor: DispatchTensor,
        param_id: DistributedParamId,
    ) -> DispatchTensor {
        let DispatchTensor {
            kind,
            checkpointing,
        } = tensor;

        let kind = match kind {
            DispatchTensorKind::Autodiff(inner_kind) => match *inner_kind {
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => {
                    DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Cuda(
                        crate::BackendTensor::Autodiff(Autodiff::<Cuda>::set_distributed_params(
                            tensor.as_autodiff().clone(),
                            param_id,
                        )),
                    )))
                }
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => {
                    DispatchTensorKind::Autodiff(Box::new(DispatchTensorKind::Remote(
                        crate::BackendTensor::Autodiff(Autodiff::<Remote>::set_distributed_params(
                            tensor.as_autodiff().clone(),
                            param_id,
                        )),
                    )))
                }
                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
                other => {
                    panic!("Distributed operations are not supported for tensor kind {other:?}")
                }
            },
            _ => panic!("Requires autodiff tensor."),
        };

        let checkpointing = if let Some(strategy) = checkpointing {
            Some(strategy)
        } else {
            Some(crate::CheckpointingStrategy::None)
        };
        DispatchTensor {
            kind,
            checkpointing,
        }
    }

    #[allow(unused_variables)]
    fn distributed_params(tensor: &DispatchTensor) -> Option<DistributedParams> {
        let DispatchTensor {
            kind,
            checkpointing: _,
        } = tensor;

        match &kind {
            DispatchTensorKind::Autodiff(inner_kind) => match &**inner_kind {
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => {
                    tensor.as_autodiff().node.distributed_params.clone()
                }
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => {
                    tensor.as_autodiff().node.distributed_params.clone()
                }

                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
                // Backends without distributed support never carry distributed params.
                _ => None,
            },
            _ => panic!("Requires autodiff tensor."),
        }
    }

    #[allow(unused_variables)]
    fn is_distributed(tensor: &DispatchTensor) -> bool {
        let DispatchTensor {
            kind,
            checkpointing: _,
        } = tensor;

        match &kind {
            DispatchTensorKind::Autodiff(inner_kind) => match &**inner_kind {
                #[cfg(feature = "cuda")]
                DispatchTensorKind::Cuda(tensor) => {
                    tensor.as_autodiff().node.distributed_params.is_some()
                }
                #[cfg(feature = "remote")]
                DispatchTensorKind::Remote(tensor) => {
                    tensor.as_autodiff().node.distributed_params.is_some()
                }

                DispatchTensorKind::Autodiff(_) => {
                    panic!("Autodiff should not wrap an autodiff tensor.")
                }
                // Backends without distributed support are never distributed.
                _ => false,
            },
            _ => panic!("Requires autodiff tensor."),
        }
    }
}

// NOTE: placeholder for autodiff module requirements
#[cfg(not(feature = "autodiff"))]
impl AutodiffBackend for Dispatch {
    type InnerBackend = Dispatch;

    type Gradients = bool;

    fn backward(_tensor: DispatchTensor) -> Self::Gradients {
        unimplemented!("Requires `autodiff` feature")
    }

    fn grad(_tensor: &DispatchTensor, _grads: &Self::Gradients) -> Option<DispatchTensor> {
        unimplemented!("Requires `autodiff` feature")
    }

    fn grad_remove(
        _tensor: &DispatchTensor,
        _grads: &mut Self::Gradients,
    ) -> Option<DispatchTensor> {
        unimplemented!("Requires `autodiff` feature")
    }

    fn grad_replace(_tensor: &DispatchTensor, _grads: &mut Self::Gradients, _grad: DispatchTensor) {
        unimplemented!("Requires `autodiff` feature")
    }

    fn inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn int_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn bool_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn q_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn from_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn int_from_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn bool_from_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }

    fn q_from_inner(_tensor: DispatchTensor) -> DispatchTensor {
        unimplemented!("Requires `autodiff` feature")
    }
}

impl Dispatch {
    /// List all available devices of the specified [type id](DispatchDeviceId).
    pub fn enumerate(type_id: DispatchDeviceId) -> Vec<DispatchDevice> {
        // TODO: right now this assumes `type_id = 0`, but WgpuDevice and LibTorchDevice have other types.
        match type_id {
            #[cfg(feature = "cpu")]
            DispatchDeviceId::Cpu => vec![CpuDevice.into()],
            #[cfg(feature = "cuda")]
            DispatchDeviceId::Cuda => (0..Cuda::device_count(0))
                .map(|i| CudaDevice::new(i).into())
                .collect(),
            #[cfg(feature = "metal")]
            DispatchDeviceId::Metal => (0..Metal::device_count(0))
                .map(|i| DispatchDevice::Metal(WgpuDevice::DiscreteGpu(i)))
                .collect(),
            #[cfg(feature = "rocm")]
            DispatchDeviceId::Rocm => (0..Rocm::device_count(0))
                .map(|i| RocmDevice::new(i).into())
                .collect(),
            #[cfg(feature = "vulkan")]
            DispatchDeviceId::Vulkan => (0..Vulkan::device_count(0))
                .map(|i| DispatchDevice::Vulkan(WgpuDevice::DiscreteGpu(i)))
                .collect(),
            #[cfg(feature = "wgpu")]
            DispatchDeviceId::Wgpu => (0..Wgpu::device_count(0))
                .map(|i| DispatchDevice::Wgpu(WgpuDevice::DiscreteGpu(i)))
                .collect(),
            #[cfg(feature = "webgpu")]
            DispatchDeviceId::WebGpu => (0..WebGpu::device_count(0))
                .map(|i| DispatchDevice::WebGpu(WgpuDevice::DiscreteGpu(i)))
                .collect(),
            #[cfg(any(feature = "flex", default_backend))]
            DispatchDeviceId::Flex => vec![FlexDevice.into()],
            #[cfg(feature = "ndarray")]
            DispatchDeviceId::NdArray => vec![NdArrayDevice::Cpu.into()],
            #[cfg(feature = "tch")]
            DispatchDeviceId::LibTorch => (0..LibTorch::device_count(0))
                .map(|i| LibTorchDevice::Cuda(i).into())
                .collect(),
            #[cfg(feature = "remote")]
            // Remote devices are keyed by a network address, which the type-id-only
            // `enumerate` can't carry. Use [`Dispatch::enumerate_remote_websocket`] to list the devices
            // behind a given address.
            DispatchDeviceId::Remote => Vec::new(),
            _ => unreachable!("No backend feature enabled."),
        }
    }

    /// List every device hosted by the remote server at `address`.
    ///
    /// Unlike [`enumerate`](Self::enumerate), remote devices are identified by a network
    /// address rather than enumerable local hardware, so they need a dedicated entry point.
    /// Connecting to the server (required to learn its device count) happens here; see
    /// [`RemoteDevice::enumerate_websocket`].
    ///
    /// Websocket-only: Iroh peers are addressed by endpoint identity, not a URL string.
    #[cfg(feature = "remote-websocket")]
    pub fn enumerate_remote_websocket(address: &str) -> Vec<DispatchDevice> {
        RemoteDevice::enumerate_websocket(address)
            .into_iter()
            .map(DispatchDevice::Remote)
            .collect()
    }
}