docs: public void printSparseFeatureMatrix(PrintWriter pw) {
      String sep = "\t";
      for (int i = 0; i < size; i++) {
        pw.print(labelIndex.get(labels[i]));
        int[] datum = data[i];
        for (int j : datum) {
          pw.print(sep + featureIndex.get(j));
        }
        pw.println();
      }
    }