Minor style issues caused by yapf

[astonzhang]

Thanks to https://github.com/d2l-ai/d2l-book/commit/06ce6657f279af162da845dd3d771ae6a61f1700, now D2L replaces fluent alias (e.g., d2l.reshape(a, (1,2)) -> a.reshape((1,2))) correctly!

Due to the use of yapf in the code prettification step, the following are new minor style issues:

• d2l.plt.scatter(d2l.numpy(features[:, 1]), d2l.numpy(labels), 1); will become d2l.plt.scatter(features[:, (1)].asnumpy(), labels.asnumpy(), 1), where the trailing semicolon (useful for showing figures only in notebooks) is trimmed and 1 is surrounded with unnecessary parentheses.

• yapf may unnecessarily produce two empty lines instead of one empty line (space inefficient for physical books):

• yapf may unnecessarily produce empty lines within our import blocks (space inefficient for physical books):

[mli]

2) is fixed by https://github.com/d2l-ai/d2l-book/commit/486cb8d2473330d200cee86efceed8d66f26804b

[mli]

3. is fixed by https://github.com/d2l-ai/d2l-book/commit/caca84a75ae014c9ef84d7ce91dde38c92860507

[mli]

1. is fixed by https://github.com/d2l-ai/d2l-book/commit/6e7a7afd57f1d6c376e4350f4784627b6de29e72