def _get_row_sparse(self, arr_list, ctx, row_id):
        """ Get row_sparse data from row_sparse parameters based on row_id. """
        # get row sparse params based on row ids
        if not isinstance(row_id, ndarray.NDArray):
            raise TypeError("row_id must have NDArray type, but %s is given"%(type(row_id)))
        if not self._trainer:
            raise RuntimeError("Cannot get row_sparse data for Parameter '%s' when no " \
                               "Trainer is created with it."%self.name)
        results = self._check_and_get(arr_list, ctx)

        # fetch row sparse params from the trainer
        self._trainer._row_sparse_pull(self, results, row_id)
        return results
Get row_sparse data from row_sparse parameters based on row_id.