default construct rpc agent options based on the backend type (#30201)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30201
Provide a default constructor so that users don't have to construct
RPC agent options. Also rename this to RPCBackend Options as suggested.
ghstack-source-id: 94411768
Test Plan: Unit tests pass.
Differential Revision: D18628698
fbshipit-source-id: 81fb45f124ad1006e628f6045162308093c9d446
