[Feature] Support Pipeline Parallism in torchrun SPMD offline inference for V1 (#17827)

Signed-off-by: Lucia Fang <fanglu@fb.com>

vllm/engine/arg_utils.py

@@ -1383,9 +1383,10 @@ class EngineArgs:
             return False
 
         if (self.pipeline_parallel_size > 1
-                and self.distributed_executor_backend not in ["ray", "mp"]):
+                and self.distributed_executor_backend
+                not in ("ray", "mp", "external_launcher")):
             name = "Pipeline Parallelism without Ray distributed executor " \
-                    "or multiprocessing executor"
+                    "or multiprocessing executor or external launcher"
             _raise_or_fallback(feature_name=name, recommend_to_remove=False)
             return False