Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'utterance_id') and self.utterance_id is not None:
            _dict['utterance_id'] = self.utterance_id
        if hasattr(self, 'utterance_text') and self.utterance_text is not None:
            _dict['utterance_text'] = self.utterance_text
        if hasattr(self, 'tones') and self.tones is not None:
            _dict['tones'] = [x._to_dict() for x in self.tones]
        if hasattr(self, 'error') and self.error is not None:
            _dict['error'] = self.error
        return _dict