* fix softmax with cross entropy out of bound; test=develop (#25549)

* fix index error; test=develop

add try..catch...

* add user defined func test=develop

* update test=develop

* update test=develop

* fix name conflicts test=develop

* add unittest test=develop

* change 2018 to 2020 test=develop

* add comment test=develop

* add comment for function test=develop

* fix details test=develop

* fix details test=develop
Co-authored-by: NLiufang Sang <slf12thuss@163.com>

* attempt to resolve tls problem, test=develop

* add glibc version check, test=develop

* fix regex, test=develop

* refine get_libc_ver, test=develop

* refine get_libc_ver, test=develop

cherry-pick,fix optimizer.state_dict and LRScheduler.state_dict to save/load dygraph

add new API:LambdaDecay,test=develop 

* enable load_program_state run in imperative mode; test=develop

* remove useless code; test=develop

* fix concat shape error
test=develop

* make default_collate_fn visible. test=develop. test=release/1.8

test=release/1.8

* test=release/1.8, cherry fix fluid.embedding

test=release/1.8

* CHERR-PICK1.8,add base class of LearningRateEpochDecay, and API: MultiStepDecay, and API: StepDecay,test=release/1.8

* fix unittest to add coverage,test=develop

* add DistributedSampler and ColorJitter, test=develop

* test=release/1.8, add index sample

test=release/1.8, test=document_fix

* fix bug of prelu when rank not equal 4, test=develop

* fix prelu inference, test=develop

* fix api, test=develop

* fix shape when mode is chennel, test=develop

* remove debug code, test=develop

* add unittest, test=develop

replace join to terminate, test=develop

* fix beam_search infershape, test=develop

* fix beam search op unittest, test=develop

fix sample code error of fluid.dygraph.NaturalExpDecay

Fix the bug for elementwise_div op, when the first var is scalar; Use the shape 1 replace the -1 in shape

添加loss自适应的学习率衰减策略。

* cherry-pick #24977

* raise index error when slice out of range; test=develop

* add uni test; test=develop

* fix format error; test=develop

* add comment for py::index_error; test=develop

* polish error message; test=develop

* polish error message; test=develop