[3/n] [torch/elastic] Introduce `EtcdRendezvousBackend`. (#55637)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55637
This diff introduces the `EtcdRendezvousBackend` type that will serve as an experimental alternative to the existing `EtcdRendezvousHandler`.
The major advantage of `EtcdRendezvousBackend` is that it delegates the bulk of the rendezvous handling logic to `DynamicRendezvousHandler` which is shared with `C10dRendezvousBackend` (see D27654492) and any other potential future rendezvous backend (e.g. Amazon S3).
ghstack-source-id: 126312209
Test Plan: Run the existing and newly-introduced unit/integration tests.
Reviewed By: tierex
Differential Revision: D27654498
fbshipit-source-id: f3259adfc9068b7e323b947a7d8d52fcd0b8ada1
