[Bugfix] Clear engine reference in AsyncEngineRPCServer (#7618)

Signed-off-by: Rui Qiao <ruisearch42@gmail.com>

vllm/entrypoints/openai/rpc/server.py

@@ -38,6 +38,8 @@ class AsyncEngineRPCServer:
         self.socket.close()
         self.context.destroy()
         self.engine.shutdown_background_loop()
+        # Clear the engine reference so that it can be GC'ed.
+        self.engine = None
 
     async def get_model_config(self, identity):
         """Send the ModelConfig"""