Return a json dictionary representing this model.

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