* fix chain doc, test=develop, test=document_preview

* refine some reader API

* import paddle.fluid, test=develop

* follow comment, test=develop

* fix the doc format of io.chain.

* Rewrite the content of the chain in Reader_cn.rst.

* modify fluid.rst and layers.rst and gen_doc.py

* synchronize api_cn of develop 0823,test=document_preview

* update io_cn.rst

* delete ComposeNotAligned_cn.rst