* enrich tutorial of InputSpec

* fix typo

* fix indent

* 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 cn doc of trace API

* modified sample code of add_position_encoding to 2.0

* polish train eval faq

* polish details

* fix path error

* add check need build paddle

* remove set -e

* add cn doc and en doc check

* fix path bug

* 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

* add cn doc of trace API

* modified doc of warpctc  ctc_loss and CTCLoss, add support float64 of log_probs

test=develop

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

* up, test=develop;test=document_fix

* 2.0.0-beta infer lib

* rename infer lib to paddle inference

* fix some docs

* fix links

* fix sample code

* 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

* optimize some words in image_segmentation/hight_level_api/getting_started

* fix image url error in image_segmentation

* link fix

* upgrade guide added

* update migration tool page

* minor change

* minor change

* update codes and format

* no ci for upgrade_guide

* minor

* minor

* 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

* polish data read faq

* polish details

* remove static mode fag

* add fleet cn_doc;test=develop

We think we should split the usage and architecture because of clear organization and future extension (we may add more usage guide)

* fix windows pip install

* change =

* change =

* fix die link

* fix