[22/n] [torch/elastic] Introduce a new from_backend static constructor for DynamicRendezvousHandler (#57150)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57150
This PR refactors the `__init__` method of `DynamicRendezvousHandler` to a `from_backend` static constructor for easier testing and future extensibility.
ghstack-source-id: 127685183
Test Plan: Run the updated unit tests.
Reviewed By: tierex
Differential Revision: D28060336
fbshipit-source-id: b07dcbb61e8ff5a536b7b021cd50438010c648dd
