* 1.Fix api sum:(1) input->sum; (2)dim->axis; (3)keep_dim->keepdim.

* 2. fix bug when len(axis) == len(x.shape). 

test=develop

* add Sampler/SequenceSampler/RandomSampler. test=develop

* add SyncBatchNorm,test=develop

* logsigmoid and LogSigmoid, test=develop

* add elu gelu relu, test=develop

* update to_variable to to_tensor, test=develop

* address review comments, test=develop

* address review comments, test=develop

* change to_variable to to_tensor in test, test=develop

* add check if fluid.data() variable no feed data, test=develop

* Add testcase for feed check, test=develop

* Conv2d_transpose and mul support channnelwise quantization, test=develop
* Skip collecting out threshold for output tensor of which the type is not fp32 or fp64, test=develop
* Fix error in test_user_defined_quantization, test=develop
* Add depthwise_conv_bn_fuse, test=develop
* Add conv_transpose_bn_fuse_pass for post_training_quant, test=develop

align expand_as op to expand.

test=develop

* add set/get default dtype

* enable mkldnn layout conversion

* review fix: remove tmp_place

* fix test mkldnn swish

* add UT for PrepareData CPU->MKLDNN

* add #ifdef PADDLE_WITH_MKLDNN

* Force-push commit
Co-authored-by: Ngrygielski <adam.grygielski@gmail.com>

add nn.functional.normalize API for API2.0, fix L1Loss API typo, test=develop

* add feature to fleet2.0 role_maker, distribute_strategy, test=develop

* add check approval
test=develop

* make fleet_localsgd_meta_optimizer work

* fix bug in localsgd meta optimizer

* set default value to strategy in distributed_optimizer test=develop

* print user-friendly error message

* adjust error sumary

set default python3,fix incompatible,cache dir for third party,unify error code,for windows (#26178)

* set default python3 for paddle windows,test=win

* set default python3,cache dir for third party,error code,test=win

* fix some incompatible

* fix some error

* set virtual environment,test=win

* add nll loss API, update demo code of the comment

* upate the target to label, test=develop

* Update the code for the margin_ranking_loss, test=develop

Update the sigmoid, Sigmoid layer for the api2.0

* Fix build whl package

* del timestamp

* fix

* Update `paddle.add`

remove `alpha`
update doc sample code
remove `:alias` etc

* Remove `imperative` reference from docstring