* update cluster train v2 doc

* WIP cluster train doc

* update

* cluster train doc

* add TOC for en doc

* fix sphix build issue

* fix error links

* fix link errors

* fix image link

* polish cluster train docs

* update general distributed training document

* fix sphinx compile error

* fix doc image error

* Create prune.md

* modification based on comment

* remove insertion

* rename id to block_id

* Update prune.md

* formatting

* Add initail design doc for regularization

* Updating image links

* Commiting the images for the equations

* Adding computation graph images

* Adding section on computation graph

* Update design doc for Python Layer

* Update document

* Design doc of SelectedRows

* Follow comments

* Update protobuf message

* Follow comments, seperate LoDTensorDesc and SelectedRows Desc

* init optimizer design

* fix index

* optimize the interface

* add a link to python_api.md

* optimize the code of Optimizer

* Update program.md
* Update
* Update

* fix block syntax

* add builder

* fix argument names

* uniform all Var to var

* fix c demo

* factor set_outputs

* update block

* delete as_ifelse_input interface from block design

* simplify set_outputs and output_num

* make the c++ codes compatible with demo

* fix compatible

* fix syntax

* update ie

* update syntax