message: Pass Config by const reference when checking its contents.
  This avoids reallocating and copying all of the vectors in the struct.
diff: @@ -926,7 +926,7 @@ inline void ReportUnexpectedDataType(DataType dtype) {
  << "in variable that should have been checked by CheckConfigDataType().";
  }
  -Status CheckConfigDataTypes(Config config) {
  +Status CheckConfigDataTypes(const Config& config) {
  // Check config so we can safely CHECK(false) in switches on config.*.dtype
  for (auto& c : config.sparse) {
  TF_RETURN_IF_ERROR(CheckConfigDataType(c.dtype));
  