Struct WhereRule 

pub struct WhereRule;

Backward rule for Op::Where (ternary select).

Forward: out[i] = if cond[i] != 0 { a[i] } else { b[i] }. Inputs: (cond, a, b) where cond is U8, a and b share dtype T.

Gradients:

• cond: None (U8 mask is non-differentiable).
• a: upstream * cast(cond, T) — gradient flows only at the slots where a was picked.
• b: upstream * cast(1 - cond, T) — gradient flows only at the slots where b was picked.

The 1 - cond mask is built via Op::AddScalar(-1.0) followed by Op::Neg: m_b = -(m_a - 1) = 1 - m_a. This avoids needing a Const(1) factory inside the rule (which would require synthesizing a slot-populated leaf via the device handle).

Trait Implementations

impl GradientRule for WhereRule

fn backward( &self, graph: &SharedGraph, _op: &Op, inputs: &[NodeId], _output: NodeId, upstream: NodeId, ) -> GradientList

Build gradient nodes for this op.