Support remote for builtin operators in distributed autograd (#28630)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28630
This includes:
1. Respect autograd context in rpc.remote for builtin ops
2. Force setting autograd context in RRef.to_here() even if the
message for to_here() does not contain any tensor.
Test Plan: Imported from OSS
Differential Revision: D18138562
Pulled By: mrshenli
fbshipit-source-id: a39ec83e556d19130f22eb317927241a017000ba
