[bugfix] add seed in torchrun_example.py (#15980)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/config.py

@@ -761,6 +761,12 @@ class ModelConfig:
         self,
         parallel_config: "ParallelConfig",
     ) -> None:
+
+        if parallel_config.distributed_executor_backend == "external_launcher":
+            assert self.seed is not None, (
+                "Seed must be set when using external launcher backend to "
+                "make sure sampling results are the same across workers.")
+
         total_num_attention_heads = getattr(self.hf_text_config,
                                             "num_attention_heads", 0)
         tensor_parallel_size = parallel_config.tensor_parallel_size