[torch/elastic] Introduce `PeriodicTimer` (#55919)
Summary:
This PR introduces a basic timer type that periodically calls a specified function. Its main use in the upcoming `DynamicRendezvousHandler` implementation will be to send periodic keep-alive updates in a background thread.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55919
Reviewed By: tierex
Differential Revision: D27740823
Pulled By: cbalioglu
fbshipit-source-id: e46fc848ab033995946a38a29c01d67d387a4cf5
