The Adam in 5-1.Transformer should be replaced by SGD

Line 202 :
`optimizer = optim.Adam(model.parameters(), lr=0.001)`

In practice, I think the effect of Adam is quite bad. When epoch = 10, cost is 1.6; when epoch = 100 or 1000, cost is still equal to 1.6.
So I think we can change Adam to SGD, that is, `optimizer = optim.SGD(model.parameters(), lr=0.001)`

Here are the effects of using SGD：
```
Epoch: 0100 cost = 0.047965
Epoch: 0200 cost = 0.020129
Epoch: 0300 cost = 0.012563
Epoch: 0400 cost = 0.009101
Epoch: 0500 cost = 0.007131
Epoch: 0600 cost = 0.005862
Epoch: 0700 cost = 0.004978
Epoch: 0800 cost = 0.004325
Epoch: 0900 cost = 0.003823
Epoch: 1000 cost = 0.003426
```

Open the 5-1.Transformer notebook and inspect line 202, where the optimizer is configured. Run the training example before and after the requested optimizer change, and confirm that the cost decreases over later epochs rather than remaining at 1.6.

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