code: def _list_fields(self):
          
          opts = {'model': self.__proxy__, 'model_name': self.__name__}
          response = _turicreate.extensions._nearest_neighbors.list_fields(opts)
  
          return sorted(response.keys())
docs: 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.