Update the metrics if not `train`

def on_batch_end(self, last_target, train, **kwargs):
        "Update the metrics if not `train`"
        if train: return
        bs = last_target.size(0)
        for name in self.names:
            self.metrics[name] += bs * self.learn.loss_func.metrics[name].detach().cpu()
        self.nums += bs