[Bugfix] Fix qwen-moe packed_modules_mapping (#26634)

Signed-off-by: Jee Jee Li <pandaleefree@gmail.com>

vllm/model_executor/models/qwen3_moe.py

@@ -634,11 +634,7 @@ class Qwen3MoeForCausalLM(
             "q_proj",
             "k_proj",
             "v_proj",
-        ],
-        "gate_up_proj": [
-            "gate_proj",
-            "up_proj",
-        ],
+        ]
     }
 
     fall_back_to_pt_during_load = False
@@ -649,6 +645,14 @@ class Qwen3MoeForCausalLM(
         quant_config = vllm_config.quant_config
         self.config = config
         self.quant_config = quant_config
+        # Only perform the following mapping when Qwen3MoeMLP exists
+        if getattr(config, "mlp_only_layers", []):
+            self.packed_modules_mapping["gate_up_proj"] = (
+                [
+                    "gate_proj",
+                    "up_proj",
+                ],
+            )
         self.model = Qwen3MoeModel(
             vllm_config=vllm_config, prefix=maybe_prefix(prefix, "model")
         )