message: Improve Cardinality() of TakeDatasetOp
  When the input dataset has finite outputs and count=-1, Cardinality()
  shuold return the cardinality of the input dataset instead of -1.
diff: @@ -67,7 +67,10 @@ int64 TakeDataset::Cardinality() const {
  }
  if (n == kInfiniteCardinality) {
  return count_;
  + } else if (count_ == kInfiniteCardinality) {
  + return n;
  }
  +
  return std::min(n, count_);
  }
  