[torch/elastic] Refactor rendezvous store initialization logic (#58057)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58057
This PR refactors the store initialization logic and moves it to the `create_backend` function for both C10d and etcd backends.
ghstack-source-id: 128671579
Test Plan: Run the existing and revised tests.
Reviewed By: tierex
Differential Revision: D28356587
fbshipit-source-id: caf9416ab811eefe4834268d8a11a48f2236ed5b
