* add clip_by_norm on kunlun, *test=kunlun

* opt matmul and matmul_v2 on kunlun, *test=kunlun

* optimize unity build, test=develop

* fix compilation error on Windows, test=develop

* fix compilation error, test=develop

* fix code style error, test=develop

update RandomHorizontal Flip random description and examples

* Optimized the adaptive_avg_pool2d op when output_size == 1

* [Custom OP]add PD_THROW and PD_CHECK for User error message

* PD_THROW and PD_CHECK, fix comment

* fix Windows error message

* fix Windows error message

* fix CI

* fix eval_if_exist_else_none bug

* fix typo

* fix typo

* fix test_op_num unittest

* remove remove_unsupport_dtype

* remove remove_unsupport_dtype

* remove test dtype

* add more include

* change dtype.h's enum as enum class to avoid conflict with inference lib

* make enum as enum class

* remove additional test

* merge develop

* polish code

* Support Incremental compilation and Add Version management

* replace hash with hashlib

* split build op marco & polish details

* revert register api del

* fix other unittest

* add unittest for static/dygraph/dy2stat

* add PE unittet

* remove usless code

* add unittest in CMakeList.txt

Change-Id: I6210ce9c60bed48f3323c47b16500302b66cedf2

* add int pad support for Pad1D/2D/3D

* fix type

* fix format

* add cache for VariableWrapper

* modify args names and vlog level

* format code style

* add log when set cache to variable_wrapper

* add log when set cache to variable_wrapper

* add comment to variableWrapper cache

* format code style

* add cpp_extension en doc

* remove cuda_cflags and add optional in doc

* refine style

* fix indent problem

* add default None

* add simple attr support and test

* add int, float attr support

* support other attribute

* add custom attrs test in cmake

* polish details

* fix test failed

* add backward test

* update test flags