[18/n] [torch/elastic] Introduce _RendezvousCloseOp (#57146)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57146
This PR introduces the `_RendezvousCloseOp` type that represents a rendezvous close operation to be executed via a `_RendezvousOpExecutor`.
ghstack-source-id: 127684991
Test Plan: Run the existing and new unit tests.
Reviewed By: tierex
Differential Revision: D28059693
fbshipit-source-id: 6c944d3b4f6a6ed2057ea2921ae8a42609998dd2
