[torch/elastic] Rename etcd-/c10d-experimental to etcd-v2 and c10d (#57764)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57764
As discussed offline this PR renames etcd-experimental backend to etcd-v2 and c10d-experimental backend to c10d.
ghstack-source-id: 128342523
Test Plan: Run the existing unit tests.
Reviewed By: kiukchung
Differential Revision: D28263739
fbshipit-source-id: c3409037ecea5a8ff6daadeeb1f2fb4205cc3852
