test=release/0.15.0

* Update build_and_install_lib_cn.rst

* Update build_and_install_lib_cn.rst

test=release/0.15.0

* fix docs dead links for 0.15.0

* Update README.cn.md

* Update visualdl.md

test=release/0.15.0

test=release/0.15.0

* hide misc APIs

* update

* fix transformer test

* update API.spec

Reduce one level of inheritence.

* Add Preface

* Add demo code

* Save file

* Refine code

* seems can work

* use elementwise strategy

* Use ElementwiseComputeEx

* Add comments

* extract functions from operator

* Refine code

* Follow comment

* code refine

* follow comments

* follow comments

add doc

* Add new_doc to develop branch

* Add pics of Anakin tutorial

* Update anakin_tutorial.md

* Delete anakin_fm_en.png

* Delete contri1.JPG

* Delete contri2.JPG

* Update build_and_install_lib_cn.rst

* Update compile_paddle_lib_cn.md

* Update compile_paddle_lib_en.md

* Update install_doc.rst

* Update install_doc.rst

* expose rank_loss python api

* Update index_en.rst

* Update index_en.rst

* Update index_en.rst

* fix build doc

* add some description about Python ABI

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

* Design doc of fixed-point quantization.
* Update fixed point quantization desigin doc.
* Fix doc format.
* Update the backward part.
* Fix the grammatical.