* add retry for prefetch

* Revert "Fix/sync barrier (#25016)"

This reverts commit be6a315f.

* reopen dist UT, test=develop

* remove fl UT, test=develop

test=develop

* fix test_multiprocess_dataloader_exception failed on CPU only version. test=develop

* clean __str__ of VarBase and ParamBase, test=develop

* clean to_string, test=develop

* update unittest, test=develop

* fix bn & in in dy, test=develop

* update instance_norm,test=develop

* fix bugs,test=develop

* add more case in unittest,test=develop

* fix,test=develop

* fix,test=develop

* add device_worker for pipeline, test=develop

As the title

As the title

* 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

* 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

* 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

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* 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

Support Various-Length Return Grammar in Dy2stat. This PR is a follow-up of https://github.com/PaddlePaddle/Paddle/pull/25176 .

The basic idea is putting no-value placeholder variables at `return` statement to make all `return` statement have same length, after that the static graph can have fixed fetch output (code at return_transformer.py). Then remove those no-value placeholder when we finally return dygraph result (code at partial_program.py).

However, various length return in Bert model is still not supported. The dy2stat can change the code as I wish but some ops which check shape at compile time (e.g. Reshape, MatMul) will throw error because of the no-value-placeholder may not have the required shape. Is this a matter? To me, those no-value placeholder will be replaced as really values meeting shape requirements at run time, so I think the solution should be some way to do the compile-time checking. By the way, every time when we have dynamic shape, it often causes problem in dy2stat. We should find a way to handle it in the future.

Fixing various return in Bert is my TODO thing and I will also find some other existing models for verification.

* fix GPU diff test=develop

* refine code test=develop