public function variance() : ColumnVector
    {
        return $this->subtract($this->mean())
            ->square()
            ->sum()
            ->divide($this->m);
    }
public function variance() : ColumnVector
    {
        return $this->subtract($this->mean())
            ->square()
            ->sum()
            ->divide($this->m);
    }
public function variance ( ) : ColumnVector { return $ this -> subtract ( $ this -> mean ( ) ) -> square ( ) -> sum ( ) -> divide ( $ this -> m ) ; }
Compute the row variance of the matrix and return it in a tuple along
with the mean.

@return \Rubix\Tensor\ColumnVector
Compute the row variance of the matrix and return it in a tuple along with the mean .