Fix Module backward hooks for all Tensor inputs/outputs (#46163)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/598
This is BC-breaking as we now explicitly don't call the hook when there are not Tensors at the top level of the output.
This feature was not working anyways as the returned grad_input/grad_output were wrong (not respecting the output structure and wrong inputs for multi-Node Module).
This is also BC-breaking as we now report the correct gradients for `nn.Module`s that contain multiple autograd `Node`s while we use to return bad results before.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46163
Reviewed By: ailzhang, mruberry
Differential Revision: D24894180
Pulled By: albanD
fbshipit-source-id: e1b5d193d2818eb2f51e2a2722c7405c8bd13c2b
