[12/n] Rename last_keep_alives to last_heartbeats in _RendezvousState (#57141)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57141
Per feedback this PR renames `last_keep_alives` to `last_heartbeats` in `_RendezvousState`.
ghstack-source-id: 127629442
Test Plan: Run the updated unit tests.
Reviewed By: tierex
Differential Revision: D28058948
fbshipit-source-id: 0db12eac56a47a426a7a48fb5c93ac6a08b0d22e