!11994 [MS][LITE][Develop]fix reference of nullptr

From: @lx0095
Reviewed-by: @zhang_xue_tong,@hangangqiang
Signed-off-by: @zhang_xue_tong

mindspore/lite/src/cxx_api/graph/graph.cc

@@ -30,5 +30,5 @@ Graph::Graph(std::nullptr_t) : graph_data_(nullptr) {}
 
 bool Graph::operator==(std::nullptr_t) const { return graph_data_ == nullptr; }
 
-ModelType Graph::ModelType() const { return graph_data_->ModelType(); }
+ModelType Graph::ModelType() const { return kMindIR; }
 }  // namespace mindspore

mindspore/lite/src/cxx_api/graph/graph_data.h

@@ -35,8 +35,6 @@ class Graph::GraphData {
 
   std::shared_ptr<lite::Model> lite_model() { return lite_model_; }
 
-  enum ModelType ModelType() const { return kMindIR; }
-
  private:
   std::shared_ptr<lite::Model> lite_model_;
 };