Return a message instead of void from rpc udf (#25283)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25283
Return a message instead of void from rpc udf
This is to help thrift style rpc where there is no need for explicit send for a response.
We need to figure out how to solve the non-blocking callback case but don't want to block the thrift backed rpc agent implementation till then.
ghstack-source-id: 89130305
Differential Revision: D16825072
fbshipit-source-id: 75cb1c9aa5a10363b1c6b12cd21c50d7047d2268
