[19/n] [torch/elastic] Introduce _RendezvousKeepAliveOp (#57147)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57147
This PR introduces the `_RendezvousKeepAliveOp` type that represents a rendezvous keep-alive heartbeat operation to be executed via a `_RendezvousOpExecutor`.
ghstack-source-id: 127685037
Test Plan: Run the existing and new unit tests.
Reviewed By: tierex
Differential Revision: D28059733
fbshipit-source-id: 31fd8fc06f03d8f9cd21558b15a06dea7ad85bc6
