List the fields stored in the model, including data, model, and
        training options. Each field can be queried with the ``get`` method.

        Returns
        -------
        out : list
            List of fields queryable with the ``get`` method.

def _list_fields(self):
        """
        List the fields stored in the model, including data, model, and
        training options. Each field can be queried with the ``get`` method.

        Returns
        -------
        out : list
            List of fields queryable with the ``get`` method.
        """
        opts = {'model': self.__proxy__, 'model_name': self.__name__}
        response = _turicreate.extensions._nearest_neighbors.list_fields(opts)

        return sorted(response.keys())