Returns the list of classification labels for which we have any explanations.

def available_labels(self):
        """
        Returns the list of classification labels for which we have any explanations.
        """
        try:
            assert self.mode == "classification"
        except AssertionError:
            raise NotImplementedError('Not supported for regression explanations.')
        else:
            ans = self.top_labels if self.top_labels else self.local_exp.keys()
            return list(ans)