Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'custom_language_model'
                  ) and self.custom_language_model is not None:
            _dict['custom_language_model'] = self.custom_language_model
        if hasattr(self, 'speaker_labels') and self.speaker_labels is not None:
            _dict['speaker_labels'] = self.speaker_labels
        return _dict