Fix copy_ forward AD to handle broadcasting (#69592)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69592
Currently, forward AD function for`copy_` (in `VariableTypeManual`) does not handle the broadcasting case. ~EDIT: but that is not a design decision, not a bug. In this PR, we make that clear as a comment.~
Note: `broadcast_to` does not have a batching rule in core, so the ops that rely on `copy_` to broadcast will still fail batched forward grad computation.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D33020603
Pulled By: soulitzer
fbshipit-source-id: 09cb702bffc74061964a9c05cfef5121f8164814
