Convert an array produced by this classifier into an array of integer
        labels and a missing value label.

def _to_integral(self, output_array):
        """
        Convert an array produced by this classifier into an array of integer
        labels and a missing value label.
        """
        if self.dtype == int64_dtype:
            group_labels = output_array
            null_label = self.missing_value
        elif self.dtype == categorical_dtype:
            # Coerce LabelArray into an isomorphic array of ints.  This is
            # necessary because np.where doesn't know about LabelArrays or the
            # void dtype.
            group_labels = output_array.as_int_array()
            null_label = output_array.missing_value_code
        else:
            raise AssertionError(
                "Unexpected Classifier dtype: %s." % self.dtype
            )
        return group_labels, null_label