docs: public static DMatrixRMaj sumRows(DMatrixSparseCSC input , @Nullable DMatrixRMaj output ) {
          if( output == null ) {
              output = new DMatrixRMaj(input.numRows,1);
          } else {
              output.reshape(input.numRows,1);
          }
  
          Arrays.fill(output.data,0,input.numRows,0);
  
          for (int col = 0; col < input.numCols; col++) {
              int idx0 = input.col_idx[col];
              int idx1 = input.col_idx[col + 1];
  
              for (int i = idx0; i < idx1; i++) {
                  output.data[input.nz_rows[i]] += input.nz_values[i];
              }
          }
  
          return output;
      }