* add stop_gradient notes in Dygraph, test=develop

* add stop_gradient notes in Dygraph, test=develop

* polish code in Dygraph, test=develop

* polish code in Dygraph, test=develop

* polish code in Dygraph, test=develop

* add Automatic pruning in Dygraph, test=develop

* fix Dygraph cn sample code, test=develop

* fix Dygraph doc format, test=develop

new document to upload in this PR

the previous doc talks about how to run CTR training on Baidu Cloud K8S Cluster.

the new doc are modified in following points.

1.Run CTR Training on Baidu Cloud K8S + Huawei Volcano
2.Auto Split CTR model into sparse ones and dense ones.
3.Cube auto fetch Sparse model, Serving auto fetch dense model.
4.provide paddle serving client to test the ctr prediction.

* polish en doc of data_loader

* follow comments

* use fluid.data()

* refine prepare steps doc, test=develop

* replace layers.data with fluid.data, test=develop

* refine doc

* follow lanxiang's comments, test=develop

* add data-parallel model doc

* Polish DyGraph doc

Unify the guide content structure.

* fix typo in doc

* add faq for lod_tensor

* format the lod_tensor.rst

* fixed one error of api_cn

* fixed some errors of api_cn

* fixed some errors of api_cn

* fixed two errors.

* fixed two errors

* fixed one error.

* fixed one error

* fixed one error.

* fixed two errors.

* fixed one error.

* fixed two errors

* fixed two links

* synchronize api_cn with the api_en of develop.

* updated develop/api_cn

* update the api_cn of develop

* update api_cn of develop

* fixed some errors

* Delete .DS_Store

* Delete .DS_Store

* Delete .DS_Store

* Delete .DS_Store

* Delete .DS_Store

* fixed some wrong links

* fixed two links

* fixed two links and some error.

* fixed some errors.

* fixed two links

* http://newicafe.baidu.com/issue/DLTP-1771/show?from=page

* http://newicafe.baidu.com/issue/DLTP-1647/show?from=page

* http://newicafe.baidu.com/issue/DLTP-1529/show?from=page

* Update natural_exp_decay_cn.rst

* Update Reader_cn.rst

* http://newicafe.baidu.com/issue/DLTP-1907/show?from=page of develop

修改了DyGraph.md文件中加载图片的错误路径，使其与PaddleBook保持统一

* fixed one error of api_cn

* fixed some errors of api_cn

* fixed some errors of api_cn

* fixed two errors.

* fixed two errors

* fixed one error.

* fixed one error

* fixed one error.

* fixed two errors.

* fixed one error.

* fixed two errors

* fixed two links

* Update cluster_quick_start.rst

test=document_preview

test=document_preview

test=document_preview

test=document_preview

remove command argument train.py from the document, which is added by mistake.

* Update fleet_api_howto_cn.rst

* Update fleet_api_howto_cn.rst

* Update fleet_api_howto_cn.rst

* documents will be ready afterwards

not ready now

* Update index_en.rst

* Update index_cn.rst

* Update index_en.rst

* Update index_cn.rst

* Update index_en.rst

* fleet api

* fleet api

* fleet api

* improve the doc of metrics introduction

* tweak the doc of metrics introduction

* Modify errors in VisualDL usage manual
Including:
	line break errors;
	punctuation errors;
	Add explanation for each pictures.

* Modify errors in VisualDL usage manual
Both Chinese and English Documents are modified.
Including :
	line break errors.
	spell errors

* Modify the line break error in visualdl_usage_en
The Line break errors in visualdl_usage_en will make the demo program crash into errors.
I have fixed these errors.

* Modify the line break error in visualdl_usage_en.md
The errors have been fixed.

* Modify errors in VisualDL usage manual
Including:
	line break errors;
	punctuation errors;
	Add explanation for each pictures.

* Modify errors in VisualDL usage manual
Both Chinese and English Documents are modified.
Including :
	line break errors.
	spell errors

* update dygraph user guide to fit new api

* refine doc

* remove pylayer support doc

* refine words

* refine sample code

* Add usage manual for VisualDL
The usage manual will be helpful for beginners to make use of VisualDL.

Components of VisualDL are introduced in details, including:
	* Definition of Functions
	* Parameters explanation
	* Demo program for each component

* add_index

* modify write_docs_cn, write_docs_en, include specifications about python 2.7.15 and creating  virtual env with conda

* gitignore update

* revert .gitignore;  update write_docs

* icafe DLTP-1328, stress  exe.close()

* DLTP-1383 不可迭代更正

* DLTP-1404 修复动态图文档中每段代码的格式和大小都不一样

* DLTP-1390 Pyreader中文显示格式问题

* DLTP-1393 无效链接

* DLTP-1421 paddle.fluid.layers.reduce_max中文翻译有问题

* DyGraph 调整标签结构