* fix optimizer parameter is a iterator; test=develop

* fix parameter list None bug; test=develop

* use is not None; test=develop

* change list to iterable; test=develop

* refine as_lodtensor, test=develop

* fix test, test=develop

* add unittest, test=develop

* handle nested_list, test=develop

* handle nested_list, test=develop

* 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

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