Return a json dictionary representing this model.

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