def _save_stats(self, epoch_data: EpochData) -> None:
        """
        Extend ``epoch_data`` by stream:variable:aggreagation data.

        :param epoch_data: data source from which the statistics are computed
        """

        for stream_name in epoch_data.keys():
            for variable, aggregations in self._variable_aggregations.items():
                # variables are already checked in the AccumulatingHook; hence, we do not check them here
                epoch_data[stream_name][variable] = OrderedDict(
                    {aggr: ComputeStats._compute_aggregation(aggr, self._accumulator[stream_name][variable])
                     for aggr in aggregations})
Extend ``epoch_data`` by stream:variable:aggreagation data.

        :param epoch_data: data source from which the statistics are computed