fix compile

ge/graph/load/model_manager/davinci_model.h

@@ -412,7 +412,7 @@ class DavinciModel {
   ///
   uint64_t GetSessionId() const { return session_id_; }
 
-  struct ErrorMessage::Context &GetErrorContext() const { return error_context_; }
+  const struct ErrorMessage::Context &GetErrorContext() const { return error_context_; }
 
   ///
   /// @ingroup ge