[torch/elastic] Fix the edge case when no node is alive (#59663)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59663
This PR fixes an edge case bug in `DynamicRendezvousHandler` where the state of the rendezvous is not always entirely updated when one or more nodes are not alive anymore.
Test Plan: Run the existing and newly-introduced unit tests.
Reviewed By: tierex
Differential Revision: D28971809
fbshipit-source-id: ebbb6a5f2b04f045c3732d6cf0f8fdc7c2381a7c
