  explicit KeyedDenseTensorColumn(const Tensor& tensor, std::vector<int64> key)
      : tensor_(tensor) {
    std::memcpy(key_, key.data(), sizeof(key_));
  }
  explicit KeyedDenseTensorColumn(const Tensor& tensor, std::vector<int64> key)
      : tensor_(tensor) {
    std::memcpy(key_, key.data(), sizeof(key_));
  }