Function fastest_device 

pub fn fastest_device() -> Device

Highest-priority backend that is compiled in and live on this host.

Probes [DEVICE_PRIORITY] in order (TPU → CUDA → ROCm → MLX → Metal → … → CPU). Use this when you want a sensible default Session target without building a graph first. For workload-specific selection, prefer crate::cost::fastest_device_for.