process_group: wait for futher_thread join before creating new one

Author: dwancn

torchft/process_group.py

@@ -610,7 +610,10 @@ def configure(self, store_addr: str, rank: int, world_size: int) -> None:
             self._rx.close()
         if self._future_queue is not None:
             self._future_queue.put(_QUEUE_CLOSE)
-            assert self._future_queue is not None
+        # wait for the future thread to exit and then close the queue
+        if self._future_thread is not None:
+            self._future_thread.join()
+        if self._future_queue is not None:
             self._future_queue.close()
 
         ctx = mp.get_context("spawn")