Remove code and logic for old style custom autograd Function (#57357)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/30696
### Release Notes
Instantiating a custom autograd function is now deprecated. Users should call `.apply()` on the class itself because it is a static method.
--end release notes--
- There are a couple error messages that we can't entirely remove because accessing these attributes of the autograd function instance may segfault (due to cdata being nullptr). Also added a TORCH_CHECK for the name attribute which previously segfaulted.
- Error message updated to convey 1) old-style functions have been deprecated 2) this access pattern was once valid
- Updates variable -> Tensor for some error messages
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57357
Reviewed By: mrshenli
Differential Revision: D28193095
Pulled By: soulitzer
fbshipit-source-id: f021b105e9a3fd4a20d6ee3dfb6a06a8c34b10ca
