Currently the CheckpointSaver saves the check point that has the largest score value. However, this is not desirable when a lower score indicates better performance, e.g. validation loss. We can either allow the user to pass a custom score function (as in the ignite.handlers), or add a flag to control returning round(engine.state.metrics[metric_name], 4) or - round(engine.state.metrics[metric_name], 4)