code: def epoch_cb(self):
          
          metrics = {}
          metrics['elapsed'] = self.elapsed()
          now = datetime.datetime.now()
          metrics['epoch_time'] = now - self.last_epoch_time
          self.append_metrics(metrics, 'epoch')
          self.last_epoch_time = now
docs: Callback function after each epoch. Now it records each epoch time
          and append it to epoch dataframe.