model: add MTP support for Nemotron model (#26725)

* model: add MTP support for Nemotron Nano model

* model: add mtp_flags for nemotron model

* address review comments

src/models/models.h

@@ -1461,6 +1461,10 @@ struct llama_model_nemotron_h_moe : public llama_model_nemotron_h {
 
     using graph = llama_model_nemotron_h::graph;
 
+    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;
 };