[torch/elastic] Improve the implementation of `RendezvousParameters` and add its unit tests. (#146)
Summary:
Pull Request resolved: https://github.com/pytorch/elastic/pull/146
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54807
Improve the implementation and the unit test coverage of `RendezvousParameters`.
Test Plan: Run the existing and newly-introduced unit tests.
Reviewed By: kiukchung
Differential Revision: D27342444
fbshipit-source-id: 88de356c0a799844a739eb9105185bb8c1acf11f
