[torch/elastic] Revise the rendezvous handler registry logic. (#55466)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55466
Improve the implementation and the unit test coverage of `RendezvousHandlerRegistry`.
### Note
See the original diff (D27442325 (https://github.com/pytorch/pytorch/commit/df299dbd7d8241669aaac7ce07ed0c034f219b4f)) that had to be reverted due to an unexpected Python version incompatibility between the internal and external PyTorch CI tests.
Test Plan: Run the existing and newly-introduced unit tests.
Reviewed By: tierex
Differential Revision: D27623215
fbshipit-source-id: 51538d0f154f64e04f685a95d40d805b478c93f9
