[Core] Add MultiprocessingGPUExecutor (#4539)

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

vllm/executor/ray_gpu_executor.py

@@ -31,7 +31,7 @@ class RayGPUExecutor(DistributedGPUExecutor):
         assert (not self.speculative_config
                 ), "Speculative decoding not yet supported for RayGPU backend."
 
-        assert self.parallel_config.worker_use_ray
+        assert self.parallel_config.distributed_executor_backend == "ray"
         placement_group = self.parallel_config.placement_group
 
         # Disable Ray usage stats collection.
@@ -264,7 +264,7 @@ class RayGPUExecutor(DistributedGPUExecutor):
                              f"required, but found {current_version}")
 
         from ray.dag import InputNode, MultiOutputNode
-        assert self.parallel_config.worker_use_ray
+        assert self.parallel_config.distributed_executor_backend == "ray"
 
         # Right now, compiled DAG requires at least 1 arg. We send
         # a dummy value for now. It will be fixed soon.