fixing Optional[Tensor] type in autodiff (#55565)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/54783
We need to be extra careful with the pattern to legitimately use `unchecked_unwrap_optional` in autodiff.
This would at least allow us to start support `Optional[Tensor]` in autodiff, which is quite common in composite layers.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55565
Reviewed By: ejguan
Differential Revision: D27825336
Pulled By: Krovatkin
fbshipit-source-id: a8562eb10ea741effff430d7417d313b1eb53dfe
