Return a json dictionary representing this model.

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