Make sure full backward hook fire when no input requires grad (#56693)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/56380
BC-breaking note:
This changes the behavior of full backward hooks as they will now fire properly even if no input to the Module require gradients.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56693
Reviewed By: ezyang
Differential Revision: D27947030
Pulled By: albanD
fbshipit-source-id: e8353d769ba5a2c1b6bdf3b64e2d61308cf624a2
