Update selected dataframe after a completed batch
        Parameters
        ----------
        df_name : str
            Selected dataframe name needs to be modified.

def _process_batch(self, param, df_name):
        """Update selected dataframe after a completed batch
        Parameters
        ----------
        df_name : str
            Selected dataframe name needs to be modified.
        """
        if param.eval_metric is not None:
            metrics = dict(param.eval_metric.get_name_value())
            param.eval_metric.reset()
        else:
            metrics = {}
        metrics['elapsed'] = datetime.datetime.now() - self.start_time
        for key, value in metrics.items():
            if key not in self._data[df_name]:
                self._data[df_name][key] = []
            self._data[df_name][key].append(value)