StringPiece SparseTensorColumn<StringPiece>::Feature(int64 batch, int64 n,
                                                     bool strong_hash) const {
  const int64 start = feature_start_indices_[batch];
  return values_.vec<tstring>().data()[start + n];
}
StringPiece SparseTensorColumn<StringPiece>::Feature(int64 batch, int64 n,
                                                     bool strong_hash) const {
  const int64 start = feature_start_indices_[batch];
  return values_.vec<tstring>().data()[start + n];
}