[xdoctest][task 79] reformat example code with google style in ...

[xdoctest][task 79] reformat example code with google style in  `python/paddle/nn/functional/loss.py` (#56901)

* [Doctest] fix No.79, test=docs_preview

* Apply suggestions from code review

* fix gpu

* fix

* fix

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