[15/n] [torch/elastic] Introduce _RendezvousStateHolder (#56538)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56538
This PR introduces the `_RendezvousStateHolder` interface and its accompanying `_BackendRendezvousStateHolder` type that is responsible for synchronizing the local rendezvous state with the other nodes.
ghstack-source-id: 127684796
Test Plan: Run the existing and new unit tests.
Reviewed By: tierex
Differential Revision: D27892600
fbshipit-source-id: a55d884a1f9b0d742787be4dff4271e076c08962
