Allow rpc.remote to create RRef on self (#29634)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29634
This implementation supports rpc.remote to self by doing the
following steps:
1. create an owner RRef
2. add the owner RRef to owners_ in RRefContext, and keep it alive
by using RRefId as the ForkId.
3. Go through serde and insert the message to the caller's thread-pool
4. When the response message gets processed, remove the itself from
RRef fork map.
Test Plan: Imported from OSS
Differential Revision: D18445812
Pulled By: mrshenli
fbshipit-source-id: e3b9aa98962c388acbc2ce294101a236d5cb2da6
