Add test case for init_rpc_backend (#26997)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26997
Reverting accidental change in https://github.com/pytorch/pytorch/pull/26919
ghstack-source-id: 91126906
Reviewed By: zhaojuanmao
Differential Revision: D17637468
fbshipit-source-id: 9ffcf4b15b37effe6b5d5f82338ff89298c82a52
