feat: add reshape/transpose/concat backward gates

[ulises-jeremias]

reshape.v, transpose, broadcast.v, stack.v, split.v exist as core tensor ops
but autograd has no derivative gates for them. Building models with non-trivial graph
shapes cannot backprop through these operations.

Task

Add backward gates for:

1. reshape / view — gradient reshapes back to original shape
2. transpose / permute — gradient transposes back
3. concat — gradient splits along the concatenation axis
4. stack — gradient unstacks

Files

• autograd/ — new gate files