test=release/1.8

* Fix test_gast_with_compatibility.py due to the problem of gast in python3.8 (#27433)

test=develop

* fix dll load bug on windows from python3.8 (#27324)

* Support python3.8 (#26850)

* Support python3.8

test=notest

* Replace the 'spawn' start method with 'fork' start method for multiprocessing, on MacOS with python>=3.8 (#27317)

* Replace the 'spawn' start method with 'fork' start method for multiprocessing, on MacOs when python>=3.8

test=develop

* Correct the error in decorator.py (#27409)

test=develop
Co-authored-by: NZhou Wei <52485244+zhouwei25@users.noreply.github.com>

* add DISTRIBUTED_TRAINER_ENDPOINTS env for cloud

* add double grad for expand, test=develop

* add double grad to reduce sum

* add mock barrier all (#24786)

* add mock barrier all
test=develop

* fix
test=develop

* fix
test=develop

* fix
test=develop

* fix gloo error
test=develop
Co-authored-by: Nxujiaqi01 <173596896@qq.com>

* fix dataset py3 (#25012)

* fix dataset py3 error
* test=develop

* fix logger (#24682)

* fix logger of FetchHandler，which may print log twice
* test=develop

* add timeout and http store in communication (#23436)

* add timeout and http store in communication, add revert and confirm in fleet
* test=develop

* modify datanorm op test=develop (#23030)
Co-authored-by: Nxujiaqi01 <173596896@qq.com>
Co-authored-by: Nyaoxuefeng <yaoxuefeng@baidu.com>

* fix prelu for compatibility with saved model of old version

* reshape alpha

test=develop
Co-authored-by: Nchalsliu <45041955+chalsliu@users.noreply.github.com>

* fix cpuid.h not found

* fix-jetson-compile-pyramid_hash

* add more info to version.txt, test=develop (#24551)

* Fix dygraph grad bugs (#25781)

* fix double grad visitid unit; test=develop

* change name hash_pair to HashPair; test=develop

* follow comment; test=develop

* remove manual seed; test=develop

* change create_graph from True to False; test=develop

* add scale_x_y for yolo_box, test=develop

* add scale_x_y for yolo_box, test=develop

* refine eps in iou_similarity, 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

* make default_collate_fn visible. test=develop. 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

* test=release/1.8, add index sample

* 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

* fix beam_search infershape, test=develop

* fix beam search op unittest, test=develop

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。

* 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

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.

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

test=develop

* 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

* Append scale for static runner outputs (#24627)

* add scale for static runner outputs, test=develop

* fix import relation, test=develop

* remove len limit, test=develop

* remove imperative data loader place limit, test=develop (#24641)

* API(dynamic_gru, chunk_eval, BeamSearchDecoder) error message enhancement (#24513)

* dynamic_gru err_msg enhancement, test=develop

* chunk_eval err_msg enhancement and fix crf_decoding output type, test=develop

* BeamSearchDecoder err msg enhancement, test=develop

* API/OP(sequence_expand_as) error message enhancement (#23712) 
Co-authored-by: NGuo Sheng <whucsgs@163.com>