Generate a random captcha image sample
        Returns
        -------
        (numpy.ndarray, str)
            Tuple of image (numpy ndarray) and character string of digits used to generate the image

def _gen_sample(self):
        """Generate a random captcha image sample
        Returns
        -------
        (numpy.ndarray, str)
            Tuple of image (numpy ndarray) and character string of digits used to generate the image
        """
        num_str = self.get_rand(self.num_digit_min, self.num_digit_max)
        return self.captcha.image(num_str), num_str