[MTP] Validate that MTP weights are actually loaded (#35548)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

vllm/model_executor/models/deepseek_mtp.py

@@ -415,6 +415,26 @@ class DeepSeekMTP(nn.Module, DeepseekV2MixtureOfExperts):
                         weight_loader(param, loaded_weight)
             if not is_fusion_moe_shared_experts_layer:
                 loaded_params.add(name)
+
+        # Validate that weights were loaded for each expected MTP layer.
+        loaded_layers: set[int] = set()
+        for param_name in loaded_params:
+            spec_layer = get_spec_layer_idx_from_weight_name(self.config, param_name)
+            if spec_layer is not None:
+                loaded_layers.add(spec_layer)
+        for layer_idx in range(
+            self.model.mtp_start_layer_idx,
+            self.model.mtp_start_layer_idx + self.model.num_mtp_layers,
+        ):
+            if layer_idx not in loaded_layers:
+                raise ValueError(
+                    f"MTP speculative decoding layer {layer_idx} weights "
+                    f"missing from checkpoint. The checkpoint may have "
+                    f"been quantized without including the MTP layers. "
+                    f"Use a checkpoint that includes MTP layer weights, "
+                    f"or disable speculative decoding."
+                )
+
         return loaded_params
 
     def _rewrite_spec_layer_name(self, spec_layer: int, name: str) -> str: