[7/n] [torch/elastic] Rename _Rendezvous to _RendezvousState (#56535)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56535
This PR renames the `_Rendezvous` class to `_RendezvousState` in preparation of the upcoming changes.
ghstack-source-id: 126979138
Test Plan: Run the existing unit tests.
Reviewed By: H-Huang
Differential Revision: D27889894
fbshipit-source-id: 027d26aa5e1acd5bba3ad2e58b140428a4a176b2
