Return a json dictionary representing this model.

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