[BugFix] Don't start a Ray cluster when not using Ray (#5570)

vllm/config.py

@@ -616,8 +616,13 @@ class ParallelConfig:
                                      "required for multi-node inference")
                 backend = "ray"
             elif ray_found:
+                if self.placement_group:
+                    backend = "ray"
+                else:
+                    from ray import is_initialized as ray_is_initialized
+                    if ray_is_initialized():
                         from ray.util import get_current_placement_group
-                if self.placement_group or get_current_placement_group():
+                        if get_current_placement_group():
                             backend = "ray"
             self.distributed_executor_backend = backend
             logger.info("Defaulting to use %s for distributed inference",