【Hackathon No.9】 新增 logspace API 设计文档

[BrilliantYuKaimin]

增加了paddle.logspace设计文档。

[paddle-bot-old]

你的PR提交成功，感谢你对开源项目的贡献!
请检查PR提交格式和内容是否完备，具体请参考示例和模版。
Your PR has been submitted. Thanks for your contribution!
Please check its format and content. For this, you can refer to Template and Demo.

[paddle-bot-old]

PR格式检查通过，你的PR将接受Paddle专家以及开源社区的review，请及时关注PR动态。
The format inspection passed. Your PR will be reviewed by experts of Paddle and developers from the open-source community. Stay tuned.

[betterpig]

LGTM. 设计文档规范清晰，内容完整。

[paddle-bot-old]

你的PR已合入community库，请进行后续代码开发，并将代码提交至Paddle仓库。
Your PR has been merged into community repository. Please move on coding part and submit your code to corresponding repo.