Add a line with `epoch` number, `smooth_loss` and `last_metrics`.

def on_epoch_end(self, epoch: int, smooth_loss: Tensor, last_metrics: MetricsList, **kwargs: Any) -> bool:
        "Add a line with `epoch` number, `smooth_loss` and `last_metrics`."
        last_metrics = ifnone(last_metrics, [])
        stats = [str(stat) if isinstance(stat, int) else '#na#' if stat is None else f'{stat:.6f}'
                 for name, stat in zip(self.learn.recorder.names, [epoch, smooth_loss] + last_metrics)]
        if self.add_time: stats.append(format_time(time() - self.start_epoch))
        str_stats = ','.join(stats)
        self.file.write(str_stats + '\n')