code: @Override
    public MatMulOutput call(final MatMulInput input) throws Exception {
      final int index = input.getIndex();
      final Matrix<Double> leftMatrix = input.getLeftMatrix();
      final Matrix<Double> rightMatrix = input.getRightMatrix();
      final Matrix<Double> result = leftMatrix.multiply(rightMatrix);
      return new MatMulOutput(index, result);
    }
docs: Computes multiplication of two matrices.
  @param input Input which contains two matrices to multiply,
  and index of the sub-matrix in the entire result.
  @return Output which contains the sub-matrix and index of it in the entire result.
  @throws Exception If the two matrices cannot be multiplied.