Return a json dictionary representing this model.

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