[cherry-pick] modified docs of group_norm and crop_tensor#20339

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Aurelius84 merged 2 commits intoPaddlePaddle:release/1.6from

zhangting2020:cherry_pick_doc

Oct 10, 2019

Commits on Oct 9, 2019

[cherry-pick] modified doc of group_norm, test=release/1.6, test=document_fix (#20234)
zhangting2020
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[cherry-pick] modified doc of crop_tensor, test=release/1.6, test=document_fix (#20203)
zhangting2020
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