message: change device_str to const
diff: @@ -1413,7 +1413,7 @@ Status FindKernelDef(
  device_type, node_name, has_experimental_debug_info,
  experimental_debug_info, node_op, node_attrs, &reg, &was_attr_mismatch));
  if (reg == nullptr) {
  - std::string device_str = DeviceTypeString(device_type);
  + const std::string device_str = DeviceTypeString(device_type);
  Status s = errors::NotFound(
  "No registered '", node_op, "' OpKernel for ",
  device_str, " devices compatible with node ",
  