Make RpcTest re-usable by other RPC backends by using init_method to initialize a RPC backend (#27320)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27320
https://github.com/pytorch/pytorch/pull/27208/
# Problem
Other RPC backends take init_method.
# Solution
Set up init_method in rpc tests.
ghstack-source-id: 91335127
Differential Revision: D17709219
fbshipit-source-id: 3184c6e9b922a6ff9f4d1cb9abfa118b23f43eeb
