[Core] Add MultiprocessingGPUExecutor (#4539)

Co-authored-by: SAHIL SUNEJA <suneja@us.ibm.com>

vllm/engine/llm_engine.py

@@ -274,6 +274,8 @@ class LLMEngine:
         """Creates an LLM engine from the engine arguments."""
         # Create the engine configs.
         engine_config = engine_args.create_engine_config()
+        distributed_executor_backend = (
+            engine_config.parallel_config.distributed_executor_backend)
 
         # Initialize the cluster and specify the executor class.
         if engine_config.device_config.device_type == "neuron":
@@ -282,13 +284,15 @@ class LLMEngine:
         elif engine_config.device_config.device_type == "cpu":
             from vllm.executor.cpu_executor import CPUExecutor
             executor_class = CPUExecutor
-        elif engine_config.parallel_config.worker_use_ray:
+        elif distributed_executor_backend == "ray":
             initialize_ray_cluster(engine_config.parallel_config)
             from vllm.executor.ray_gpu_executor import RayGPUExecutor
             executor_class = RayGPUExecutor
+        elif distributed_executor_backend == "mp":
+            from vllm.executor.multiproc_gpu_executor import (
+                MultiprocessingGPUExecutor)
+            executor_class = MultiprocessingGPUExecutor
         else:
-            assert engine_config.parallel_config.world_size == 1, (
-                "Ray is required if parallel_config.world_size > 1.")
             from vllm.executor.gpu_executor import GPUExecutor
             executor_class = GPUExecutor