*num_retvals = TF_OperationNumOutputs(operation);
    for (int i = 0; i < *num_retvals; ++i) {
      retvals[i] = new GraphTensor({operation, i}, g_);
    }
    return absl::OkStatus();
  }

  Status SetAttrString(const char* attr_name, const char* data,
                       size_t length) override {
    tensorflow::StringPiece s(data, length);
    op_->node_builder.Attr(attr_name, s);
    return absl::OkStatus();
  }