message: Fix NgraphSerialize
diff: @@ -347,7 +347,7 @@ Status NgraphSerialize(const std::string& file_name,
  "Passed a null pointer as ng function to serialize");
  }
  try {
  - ngraph::serialize(ng_function, json_indentation);
  + serialized = ngraph::serialize(ng_function, json_indentation);
  } catch (...) {
  return errors::Internal("Failed to serialize ngraph function");
  }
  