  string GetCollectiveKey(OpKernelContext* c) {
    return CollectiveKey(c, col_params_->group.group_key,
                         col_params_->instance.instance_key);
  }
  string GetCollectiveKey(OpKernelContext* c) {
    return CollectiveKey(c, col_params_->group.group_key,
                         col_params_->instance.instance_key);
  }