[Bugfix] Fix MTP weight loading (#21941)

vllm/model_executor/models/deepseek_mtp.py

@@ -182,6 +182,8 @@ class DeepSeekMTP(nn.Module, SupportsPP):
         stacked_params_mapping = [
             ("gate_up_proj", "gate_proj", 0),
             ("gate_up_proj", "up_proj", 1),
+            ("fused_qkv_a_proj", "q_a_proj", 0),
+            ("fused_qkv_a_proj", "kv_a_proj_with_mqa", 1),
         ]
 
         expert_params_mapping = FusedMoE.make_expert_params_mapping(
@@ -212,6 +214,13 @@ class DeepSeekMTP(nn.Module, SupportsPP):
                 if (("mlp.experts." in name) and name not in params_dict):
                     continue
                 name = name.replace(weight_name, param_name)
+
+                # QKV fusion is optional, fall back to normal
+                # weight loading if it's not enabled
+                if ((param_name == "fused_qkv_a_proj")
+                        and name not in params_dict):
+                    continue
+
                 # Skip loading extra bias for GPTQ models.
                 if name.endswith(".bias") and name not in params_dict:
                     continue