Merge c10d elastic agent tests into local_elastic_agent_test.py file (#59657)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59657
Introduce tests that test elastic agent with c10d and etc2-v2 rendezvous backends.
Added a port allocation method that uses sockets to find an available port for the c10d backend. This way, agents that are created will all share the specified address/port and can communicate.
Added a method that abstracts the backend to use when running a test. This way, any tests can quickly be switched to run on the backend of choice (c10d, etcd, or etcd-v2)
Test Plan: Tests various components of the elastic agent with 3 different backends: etcd, etcd-v2, and c10d.
Reviewed By: tierex
Differential Revision: D28972604
fbshipit-source-id: fd4cff6417fefdf0de9d7a114820914b968006a8
