[21/n] [torch/elastic] Introduce _RendezvousJoinOp (#57149)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57149
This PR introduces the `_RendezvousJoinOp` type that represents a rendezvous join operation to be executed via a `_RendezvousOpExecutor`.
ghstack-source-id: 127685142
Test Plan: Run the existing and new unit tests.
Reviewed By: tierex
Differential Revision: D28059785
fbshipit-source-id: 6e67a54289eef1a2349fcc52f8841e49c139459a
