enforce rref JIT pickling to be in the scope of rpc calls (#34689)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34689
rref JIT pickling is only allowed inside rpc calls. enforcing this by adding a thread local variable isInRpcCall and set it as True when converting rpc requests or responses to message, before calling JIT::pickle(). Inside JIT::pickle(), it allowes to pickle RRef only when the isInRpcCall is true.
ghstack-source-id: 100481001
Test Plan: unit tests
Differential Revision: D20429826
fbshipit-source-id: dbc04612ed15de5d6c7d75a4732041ccd4ef3f8c
