test, fix deepcopy of tensor with grad (#50663)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/3307
Previously, `self.grad` was not ~cloned~ deepcopied to the returned tensor in `deepcopy`. Added a test and an implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50663
Reviewed By: heitorschueroff
Differential Revision: D26074811
Pulled By: albanD
fbshipit-source-id: 536dad36415f1d03714b4ce57453f406ad802b8c
