[20/n] [torch/elastic] Introduce _RendezvousExitOp (#57148)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57148
This PR introduces the `_RendezvousExitOp` type that represents a rendezvous exit operation to be executed via a `_RendezvousOpExecutor`.
ghstack-source-id: 127685094
Test Plan: Run the existing and new unit tests.
Reviewed By: tierex
Differential Revision: D28059764
fbshipit-source-id: 2da428885f1390957242fdd82d68cee2ac273c71
