Function dispatch_gradient 

pub fn dispatch_gradient(
    graph: &SharedGraph,
    op: &Op,
    inputs: &[NodeId],
    output: NodeId,
    upstream: NodeId,
) -> Option<GradientList>

Dispatch entry consulted by NodeHandle::backward before falling through to the inline match. Returns None if no rule is registered for op — the caller then handles it inline.

As ops migrate to GradientRule impls, their inline match arms are deleted and the rule is added below.