添加在非 jupyter notebook 环境下代码需要引入的包的代码。

虽然对于老鸟这不是个事，但是对于看文档学习的初学者来说，确实是件困惑的事。

Author: Jiuyong

chapter4/4.5-multiply-gpu-parallel-training.ipynb

@@ -499,6 +499,8 @@
    "outputs": [],
    "source": [
     "# 以下脚本在jupyter notebook执行肯定会不成功，请保存成py文件后测试\n",
+    "import torch\n",
+    "import argparse\n",
     "from torch.utils.data.distributed import DistributedSampler\n",
     "from torch.utils.data import DataLoader\n",
     "\n",