[Core][Bugfix][Perf] Introduce MQLLMEngine to avoid asyncio OH (#8157)

Co-authored-by: Nick Hill <nickhill@us.ibm.com>
Co-authored-by: rshaw@neuralmagic.com <rshaw@neuralmagic.com>
Co-authored-by: Robert Shaw <114415538+robertgshaw2-neuralmagic@users.noreply.github.com>
Co-authored-by: Simon Mo <simon.mo@hey.com>

vllm/executor/cpu_executor.py

@@ -106,6 +106,7 @@ class CPUExecutor(ExecutorBase):
                         )) for rank in range(1, world_size)
                 ]
 
+        self.worker_monitor = None
         if world_size != 1 or is_async:
             if is_async:
                 async_worker_list = self.workers + [self.driver_worker]