* Support assignment to a Variable in dynamic mode. Note: not deal with backward.

* Rewrite VarBase __setitem__ for high-performance.

* try to test 3 means to do __setitem__ and test the performance of 3 means.

* Retain the means of the highest performance: C++ code and don't trace op.

* add gloo initializer, test=develop

* update name_scope example code

test=document_fix

* refine name_scope doc

test=document_fix

* fix example code

* rename var to tensor

* remove static

* fix return type

* modify sample code

* variable -> tensor

* migrate program_guard sample code

* refine error message

* migrate program_guard

* refine comment style

* fix indent

* add get final strategy for user to print final strategy

* fix api alias

* remove cos_sim

* add float64 input to ctc_loss

* modified error message of  warpctc

* update repo and tag of warpctc

* add test for warpctc with float64 input

* modified warpctc.cmake to make sure build always

* resolved sample code bug of warpctc

* add core.ops in warpctc dygraph

* fix a bug of test

* 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 test_dist_fleet_heter_ctr & peformance update

* fix pool bug, test=develop

* fix coverage,test=develop

* fix bug, test=develop

* refine vision models

* add static check on data()

* follow comments

* fix ut

* 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

* [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

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

* make places of DataLoader optional. test=develop

* test=develop, bug fix