[Reducer] Remove replica size == 1 checks (#58603)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58603
No longer need these checks
ghstack-source-id: 129498227
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D28549893
fbshipit-source-id: a89bf8c3fc3aba311a70fd37e5a6aa5dc14b41b9
