enable multi-node in external launcher mode

Differential Revision: D88115795

Author: xieyangxu

vllm/config/parallel.py

@@ -593,10 +593,10 @@ def __post_init__(self) -> None:
                 "max_parallel_loading_workers is currently "
                 "not supported and will be ignored."
             )
-        if self.distributed_executor_backend not in ("mp", "uni") and self.nnodes > 1:
+        if self.distributed_executor_backend not in ("mp", "uni", "external_launcher") and self.nnodes > 1:
             raise ValueError(
                 "nnodes > 1 can only be set when distributed executor "
-                "backend is mp or uni."
+                "backend is mp, uni or external_launcher."
             )
 
     @property