[xdoctest][task 167-170] reformat example code with google style in /paddle/vision/datasets/*; test=docs_preview (#56906)

* reformat example code with google style

* udpate

* update

* add timeout for dataset download

* update cifar timeout

* update cifar timeout and fix an output

* update cifar timeout

* add a blank line

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Co-authored-by: NSigureMo <sigure.qaq@gmail.com>