  StatsPartitionKey(const int32_t node_id, const int32_t feature_dim,
                    const int32_t bucket_id)
      : node_id(node_id), feature_dim(feature_dim), bucket_id(bucket_id) {}
  StatsPartitionKey(const int32_t node_id, const int32_t feature_dim,
                    const int32_t bucket_id)
      : node_id(node_id), feature_dim(feature_dim), bucket_id(bucket_id) {}