Support RRef.backward() for local RRefs. (#46568)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46568
This PR adds support for an RRef.backward() API. This would be useful
in applications like pipeline parallelism as described here:
https://github.com/pytorch/pytorch/issues/44827
This PR only adds support for local RRefs, remote RRef support will be added in
a follow up PR.
ghstack-source-id: 115100729
Test Plan:
1) unit tests.
2) waitforbuildbot
Reviewed By: mrshenli
Differential Revision: D24406311
fbshipit-source-id: fb0b4e185d9721bf57f4dea9847e0aaa66b3e513
