* 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

* add clean up; test=develop

* add import; test=develop

PR types: Bug fixes
PR changes:Ops
Describe:
Fix the broadcast bug in elementwise ops, when elementwise ops do not have fast gradient calculate way，use the default way to calculate.

* Update sigmoid output from Y to out, test=develop (#24765)

* Collecting concat output threshold, test=develop (#24742)

* Add output threshold for ops that have several output activations, test=develop (#24726)

* [Fix bug] Init scale node in OutScaleForTrainingPass and enable test_quantization_scale_pass UT (#24393)

* Init scale node in OutScaleForTrainingPass, test=develop
* Enable test_quantization_scale, test=develop

Enhance error message of checkpoint_notify_op, fake_init_op gen_nccl_id_op and listen_and_serv_op (#24554) (#24844)

test=develop

* cherry-pick #24729

* cherry-pick #24762

* remove pow in speed up in dequantize_log test=develop

* remove pow in speed up in dequantize_log test=develop

* fix unittest test=develop

API/OP（sequence_first_step，sequence_last_step，sequence_mask，beam_search，beam_search_decode） error message enhancement ,test=release/1.8 (#24668)

* fix bug of varbase.__getitem__, test=develop (#24642)

* fix bug of varbase.__getitem__, test=develop

* fix bug of float and other type, test=develop

* fix unittest, test=develop