幸苦了！ Merge pull request #688 from ice-world/master

补全了缺失的部分代码

Author: jiangzhonglian

docs/2.0/tutorials/beginner/basics/optimization_tutorial.md

@@ -212,6 +212,18 @@ def test_loop(dataloader, model, loss_fn):
 
 我们初始化了损失函数和优化器，传递给 `train_loop` 和 `test_loop`。你可以随意地修改 epochs 的数量来跟踪模型表现的进步情况。
 
+```py
+loss_fn = nn.CrossEntropyLoss()
+optimizer = torch.optim.SGD(model.parameters(), lr=learning_rate)
+
+epochs = 10
+for t in range(epochs):
+    print(f"Epoch {t+1}\n-------------------------------")
+    train_loop(train_dataloader, model, loss_fn, optimizer)
+    test_loop(test_dataloader, model, loss_fn)
+print("Done!")
+```
+
 输出:
 
 ```py