Callback function after each epoch. Now it records each epoch time
        and append it to epoch dataframe.

def epoch_cb(self):
        """Callback function after each epoch. Now it records each epoch time
        and append it to epoch dataframe.
        """
        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