Make remaining autograd methods return futures (#57861)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57861
The very last methods left that still didn't return Futures were the autograd ones, but they're very easy to port.
We've now finished the conversion of RequestCallback to be fully Future-based!
ghstack-source-id: 129567055
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D28286173
fbshipit-source-id: 1de58cee1b4513fb25b7e089eb9c45e2dda69fcb
