Return a json dictionary representing this model.

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