[Bugfix] Fix default behavior/fallback for pp in v1 (#16057)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/engine/arg_utils.py

@@ -1521,8 +1521,9 @@ class EngineArgs:
         # PP is supported on V1 with Ray distributed executor,
         # but off for MP distributed executor for now.
         if (self.pipeline_parallel_size > 1
-                and self.distributed_executor_backend == "mp"
-                and _warn_or_fallback("PP (MP distributed executor)")):
+                and self.distributed_executor_backend != "ray"):
+            name = "Pipeline Parallelism without Ray distributed executor"
+            _raise_or_fallback(feature_name=name, recommend_to_remove=False)
             return False
 
         # ngram is supported on V1, but off by default for now.