fix docs bugs

[xiongjunfeng]

PR types

Bug fixes

PR changes

APIs

Describe

Issues: https://github.com/PaddlePaddle/docs/issues/5002 https://github.com/PaddlePaddle/docs/issues/5032

1.IPUPlace的源代码 https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/device/init.py#L81 显示这是一个function，而中文版API https://www.paddlepaddle.org.cn/documentation/docs/zh/develop/api/paddle/device/IPUPlace_cn.html 却标注为class，认为应该以源代码为准，因此修改了 docs/api/paddle/device/IPUPlace_cn.rst

2.docs/api/paddle/fft/fft_cn.rst 中在参数部分有2处错误的换行，其中将一个句号的换行可能导致了 中文版API https://www.paddlepaddle.org.cn/documentation/docs/zh/develop/api/paddle/fft/fft_cn.html#canshu 页面上参数部分，参数axis的描述被作为参数n的后续描述了。因此修改该部分错误。

现在已经可以看到预览效果，对该rst的修改解决了该问题。

[paddle-bot-old[bot]]

感谢你贡献飞桨文档，文档预览构建中，Docs-New 跑完后即可预览，预览链接：http://preview-pr-5030.paddle-docs-preview.paddlepaddle.org.cn/documentation/docs/zh/api/index_cn.html 预览工具的更多说明，请参考：[Beta]飞桨文档预览工具

[CLAassistant]

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2 out of 3 committers have signed the CLA.

:white_check_mark: Ligoml
:white_check_mark: xiongjunfeng
:x: xiong

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[Ligoml]

cla还没有完成签署，完成后可以合入pr @xiongjunfeng 和研发确认了一下，class是对的哈