Update autograd related comments (#50166)
Summary:
Remove outdated comment and update to use new paths.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50166
Reviewed By: zou3519
Differential Revision: D25824942
Pulled By: albanD
fbshipit-source-id: 7dc694891409e80e1804eddcdcc50cc21b60f822
