(torchx/local_scheduler) go back to using os.killpg in local_scheduler

Summary: Reverting the os.killpg -> os.kill part of #1062.

Reviewed By: d4l3k

Differential Revision: D74205288

Author: kiukchung

torchx/schedulers/local_scheduler.py

@@ -311,7 +311,7 @@ def terminate(self) -> None:
         """
         # safe to call terminate on a process that already died
         try:
-            os.kill(self.proc.pid, signal.SIGTERM)
+            os.killpg(self.proc.pid, signal.SIGTERM)
         except ProcessLookupError as e:
             log.debug(f"Process {self.proc.pid} already got terminated")