[2/n] [torch/elastic] Introduce `C10dRendezvousBackend`. (#55636)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55636
This diff introduces:
- The `C10dRendezvousBackend` type to support C10d stores as rendezvous backends.
- A fix to the `TCPStore.compare_set()` function to support non-existent keys.
- A placeholder `c10d-experimental` registry to instantiate C10d-baked rendezvous backends via `get_rendezvous_handler()`.
ghstack-source-id: 126312162
Test Plan: Run the existing and newly-introduced unit/integration tests.
Reviewed By: tierex
Differential Revision: D27654492
fbshipit-source-id: 09f498138b35186de4b0e174adb33fb5b5aa4b52
