Return a json dictionary representing this model.

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