Remove backward and requires_grad from Autograd backend key (#49613)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49613
Just following a TODO in the code base...
ghstack-source-id: 119450484
Test Plan: waitforsandcastle
Reviewed By: ezyang
Differential Revision: D25644597
fbshipit-source-id: 26f5fa6af480929d0468b0de3ab103813e40d78b
