[RPC] Add option to make rref.get_type not block. (#50977)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50977
Adds a `blocking` flag that can be set to False to make this API return a `Future` to the type. This is to make this function non-blocking, mostly for a future change that will allow `rref.rpc_async()` to be completely non-blocking (it currently calls and waits for this function that issues an RPC in-line).
ghstack-source-id: 121021433
Test Plan: Modified UT
Reviewed By: mrshenli
Differential Revision: D25944582
fbshipit-source-id: e3b48a52af2d4578551a30ba6838927b489b1c03
