[1/n] [torch/elastic] Introduce `DynamicRendezvousHandler` and `RendezvousBackend`. (#55635)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55635
This diff introduces the `DynamicRendezvousHandler` type as a stub implementation and its accompanying `RendezvousBackend` interface.
`DynamicRendezvousHandler` is intended to be a backend-agnostic type that will contain the core (bulk) logic of rendezvous handling. Any backend specific operation will be delegated to a concrete subclass of `RendezvousBackend` (e.g. `C10dRendezvousBackend` - see D27654492) that is passed as a constructor argument to `DynamicRendezvousHandler`.
ghstack-source-id: 126304697
Test Plan: Run the existing and newly-introduced unit/integration tests.
Reviewed By: tierex
Differential Revision: D27654478
fbshipit-source-id: 9fc89a6e4cb308971c65b29a7c5af7ae191f70c5
