[Bugfix] [Easy] Fixed a bug in the multiprocessing GPU executor. (#6770)

vllm/executor/multiproc_gpu_executor.py

 import asyncio
 import os
 import signal
+import threading
 import weakref
 from functools import partial
 from typing import Any, List, Optional
@@ -115,6 +116,7 @@ class MultiprocessingGPUExecutor(DistributedGPUExecutor):
             if executor := ref():
                 executor.shutdown()
 
+        if threading.current_thread() is threading.main_thread():
             signal.signal(signal.SIGINT, shutdown)
             signal.signal(signal.SIGTERM, shutdown)