* Fix example code of api `fluid.layers.switch_case` to use api2.0

* add alias `paddle.static.nn.switch_case`
    
* delete alias `paddle.nn.switch_case` and `paddle.nn.control_flow.switch_case`

* migrate 2.0 api doc

* migrate program_guard

* variable -> Tensor

* Fix doc and example code of api fluid.layers.case

* delete alias paddle.nn.switch_case and paddle.nn.control_flow.switch_case  

* add paddle.static.nn.switch_case

* update some Distributed Strategy doc

* update links

* refine link

* 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

test=develop

* Add example for buffered and cache, test=develop, test=document_fix

* up, test=develop;test=document_fix

* test=document, bug fix

* test=develop, add utilbase doc

* test=develop, bug fix

* test=develop, ci

* test=develop, ci

* test=develop, rm fs

* remove paddle.reader APIs. test=develop

* remove paddle.reader

* link fix

* add dygraph cn_doc;test=develop

* update doc

* add fleet meta lars and lamb api document

* add fleet meta lars and lamb api document

* test=develop

* update doc for hapi.model.save and hapi.model.Model in 2.0-RC

* fix example code for Model

* fix doc for Model example code

* fix sample code, add import InputSpec

* add fleet cn_doc;test=develop

Co-authored-by: Nroot <root@yq01-qianmo-com-255-129-13.yq01.baidu.com>

* add docs

* add Tensor concept introduction

* add Tensor concept introduction

* add Tensor concept introduction

* add Tensor concept introduction

* add Tensor concept introduction

* fix example code

* Update paddle/metrics

* Fix format

* Update Metric format

* update hapi Model doc

* add summary

* mv Model_cn.rst to model dir

* fix hapi test=develop

* fix_model test=develop

* fix gen_doc test=develop

* fix alias_mapping

* fix_gen_doc test=develop

* fix hapi test=develop

* fix model test=develop

* add formula for entropy and kl-divergence

* fix formula in cn doc

* fix formula for Normal

* add paddle.io. test=develop

* fix indent. test=develop