Enforce rref python pickling to be in the scope of RPC call (#34755)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34755
This diff disallows to use python pickler to pickle RRef. RRef can only be pickled in the scope of RPC call using _InternalRPCPickler.
ghstack-source-id: 100481337
Test Plan: unit tests
Differential Revision: D20453806
fbshipit-source-id: ebd4115ee01457ba6958cde805afd0a87c686612
