* add paddle new save load doc

* move static save apis & polish multiple details

* add api mapping

* move apis place

* fix example code error

* polish model save load guide

* polish example code

* fix details example error

* polish guide doc

* change title format

* remove history reason

* remove other history reason

* add enable_static for old save/load

* add enable_static for old api

* update the demo code and the doc of varbase.backward.

* remove the dependence of  BackwardStrategy

* modify nn.data into static.data

* Update index_cn.rst

* nn.data to static.data

* remove alias

add set_global_initializer API doc

fix the diff of sample code in chinese doc with english doc

fix chinese doc of gradient_clip 

As the title

* fix program api sample code error

* fix detail error

* fix build strategy cn doc sample code diff

* fix code error

* fix execution strategy error

* fix LayerNorm error

fix regularizaiton documention to dispel puzzle of user

* add disable_dygraph enable_dygraph en cn docs
test=develop

* clean DS_Store
test=develop

* add missing get_program_persistable_vars and get_program_parameter doc
test=develop

* add missing save doc, add link
test=develop

* remove FC, test=develop

* delete en_doc for fc, test=develop

* polish code style, test=develop

* change FC to Linear, test=develop

* update load api_cn, test=develop

* polish api comment, test=develop

* fix code bug, test=develop

* polish api comment, test=develop

* add load_op_library cn doc

* fix grammar

* fix op cn name,test=develop

Add Chinese doc of required_version， corresponding to Eng version： https://www.paddlepaddle.org.cn/documentation/docs/en/api/fluid/require_version.html#require-version

* add all parameters cn don

* add print result

* del left name_scope docs
test=develop

* minor fix
test=develop

* update gradient comment in Variable API, test=develop

* add sample code for Variable's gradient api, test=develop

test=develop

test=develop

* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* add astype api in Variable and refine code style

* refine doc

* fix DataFeedDesc set_batch_size, set_dense_slots cn api doc test=document_preview

* fix continuous_value_model op cn doc

* fix some rst error

* add cpu support only in cvm op

* add explain of N in cvm

* doc fix

* build strategy api doc