mimo2: address MTP review feedback (#26228)

Co-authored-by: tnhnyc <115956684+tnhnyc@users.noreply.github.com>

src/models/models.h

@@ -2127,6 +2127,10 @@ struct llama_model_mimo2 : public llama_model_base {
         graph(const llama_model & model, const llm_graph_params & params);
     };
 
+    struct graph_mtp : public llm_graph_context {
+        graph_mtp(const llama_model & model, const llm_graph_params & params);
+    };
+
     std::unique_ptr<llm_graph_context> build_arch_graph(const llm_graph_params & params) const override;
 };