Follow the undefined Tensor <-> None rule better in torch dispatch (#67793)
Summary:
As per title. This in particular allows to more easily override backward function for which the underlying backend returns `None`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67793
Reviewed By: zou3519
Differential Revision: D32242962
Pulled By: albanD
fbshipit-source-id: 6e114def90ee9499161e1303d301ba7fd003ff89
