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

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

* Fix example code of api fluid.layers.case

* delete paddle.nn.case alias and add paddle.static.nn.case

* support elementwise add, activation, matmul on Baidu Kunlun
* test=kunlun

* minor
* test=kunlun

* reconstuct the xpu directory
* test=kunlun

* minor
* test=kunlun

* minor
* test=kunlun

* minor
* test=kunlun

* minor
* test=kunlun

* minor
* test=kunlun

register log double grad kernel for cpu and cuda

* fix sgd/momentum/dpsgd/rmsprop error message

* refine broadcast_op_handle

* refine some error messages

* refine some files

* fix bug

* fix bug

* fix bug

* follow comments

* follow comments

* fix test_dist_fleet_heter_ctr & peformance update

* fix pool bug, test=develop

* fix coverage,test=develop

* fix bug, test=develop

fix cpplint error for the autmic max/min

* refine vision models

* add static check on data()

* follow comments

* fix ut

* refine err msg

* follow comments

This reverts commit 29f1560d.

* add xpu in heter mode
test=develop

* BOOST_CONST_GET; PADDLE_THROW
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* refine
test=develop

* refine
test=develop

* refine
test=develop

* refine code
test=develop

* fix bn double grad, test=develop

* update, test=develop

* try to fix paddle save load test

* open paddle save load

* replace dataloader

* remove dataloader

* add fp16 for matmul

add abs support double grad for the api 2.0

* refine PADDLE_ENFORCE

* [buf fix]:fix unittest test_activation_op error

* split long-time unittests to smaller ones

* rename some unittests

* fix unittest_group_norm_op_v2, test=develop

* fix unittest_group_norm_op_v2, test=develop

fix cuda atomic for ARCH<350 for the automic_max

* Add pass compatibility check for four mkldnn pass, test=develop

* modify ci api white list, add annotation

* modify ci api white list, add annotation

* move gpu_not_white list to wlist.json

* add null line

* update gcc8 in python3 ci docker

* change cuda 10.2

* update cudnn8

* nvidia error cuda10.2-cudnn8-centos6 images

* fix third cache

* add unittest count of windows

* Reduce the number of retries