As the title

fix nce sample code for python3 compatability.

* connect stop_gradient with trainable; test=develop

* add value error message; test=develop

* add paddle.fleet.DistributedStrategy for 2.0

* add Pad2D and Leaky_ReLU, test=develop

* update,test=develop

* change name,test=develop

* add unittest and redine docs,test=develop

Fix the doc example api for the scale op

* Save output threshold by argname_index, test=develop

* test=develop, fix softmax

* add cast transform and its UT for dygraph_to_static. 

* fix test_lac timeout test=develop

* disable it on windows test=develop

* modify pipeline optimizer, test=develop

* show the attr and functions of the Layer,test=develop

* add buffer for dir,test=develop

* fix __dir__,test=develop

* fix doc of Layer.__dir__, test=develop

* add mish op. test=develop

* support tuple/list init for VarBase,test=develop

* fix doc of fluid.dygraph.to_variable,test=develop

* fix doc of fluid.dygraph.to_variable,test=develop

* fix the compatibility of PY2 and PY3 in paddle.distributed.launch
test=develop

* only pull log of local rank 0
test=develop

* log exception if UnicodeEncodeError occurs when pulling log in
paddle.distributed.launch
test=develop
Co-authored-by: NSunGaofeng <peakbee@gmail.com>

* Add `matrix_nms_op`

test=develop

* Make ci happy

test=develop

* Exit early when no detection

test=develop

* Fix license year

test=develop

* Output index as well

test=develop

* Match nms2 lod behavior and add `return_index` flag

test=develop

* Make CI happy

test=develop

* Fix wording

test=develop

* disable distributed UT temporary，enable it soon, test=develop

test=develop

* fix test_multiprocess_dataloader_static random fail. test=develop

* add sentiment unittest

* add LAC model

* add test=develop

* rename tmp test=develop

* fix timeout test=develop

* undo tmp_var test=develop

* test=develop, fix_embedding

* add new API: MultiStepDecay, a new learing rate strategy, test=develop

* add new API: MultiStepDecay, a new learing rate strategy,test=develop

* add new API: MultiStepDecay, a new learing rate strategy,test=develop

* add base class of LearningRateEpochDecay, and MultiStepDecay, and StepDecay, test=develop

* fix doc to add coverage,test=develop

* Modify tmp var name prefix in dygraph test=develop

* refine comment test=develop

* add new api: optimizer.set_lr, test=develop

* add API doc and example code for optimizer.set_lr,test=develop

* add API doc and example code for optimizer.set_lr,test=develop

* Modified doc to :api_attr: imperative,test=develop