[torchelastic] Improve process termination logic (#61602)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61602
The diff introduces signal handlers and SignalException that is raised when the agent process receives SIGTERM or SIGINT.
When any of these signals received, the termination handler will raise the `SignalException`. The exception will then be processed by the main agent loop. The `shutdown(signum)` will be invoked, that would propagate the received signal to the child processes. The default 30 seconds timeout introduced: if child processes will not be able gracefully terminate during this timeout, the agent process would kill the processes via SIGKILL.
Test Plan: unittests, sandcastle
Reviewed By: cbalioglu
Differential Revision: D29671783
fbshipit-source-id: 3dbca2125676dc18d417cc3e3bb0301fdd42737a
