[16/n] [torch/elastic] Introduce _RendezvousOpExecutor (#57144)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57144
This PR introduces the `_RendezvousOpExecutor` interface. Implementers of this interface are responsible for executing rendezvous operations in a state machine that outputs actions based on the current state of the rendezvous.
ghstack-source-id: 127684898
Test Plan: None beyond `flake8` and `mypy` as this is solely an interface definition.
Reviewed By: tierex
Differential Revision: D28059159
fbshipit-source-id: 8e7da33e02336206cddbe76d773681e98c28a98f
