add default arg for init_method (#30208)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30208
Adds default arg for init_method so users don't have to pass this in,
and moves it to `RpcBackendOptions` struct. Removes `init_method` arg from rpc.init_rpc. Also fixes some docs.
ghstack-source-id: 94500475
Test Plan: Unit tests pass.
Reviewed By: mrshenli
Differential Revision: D18630074
fbshipit-source-id: 04b7dd7ec96f4c4da311b71d250233f1f262135a
