* add_three_args

* add broadcasting index and some common args text=develop

* fix broadcasting index test=develop

* fix broadcasting test=develop

* update dygraph doc; test=develop

* change dygraph to imperative; test=develop

* move impertive to beginer guid; test=develop

This reverts commit 685da70d.

* add hap readme, test=develop

* add hap readme, test=develop

ATT. 更新了中文文档中的“编程指南”，其中关于使用控制流的例子改为使用飞桨1.7版本提供的新控制流API