  std::string DebugString() const override {
    return absl::StrFormat(
        "Collective Group with group_key = %d, group_size = %d, rank = %d",
        group_key_, group_size_, rank_);
  }
  std::string DebugString() const override {
    return absl::StrFormat(
        "Collective Group with group_key = %d, group_size = %d, rank = %d",
        group_key_, group_size_, rank_);
  }