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