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

* delate some wrong message test=develop, test=document_fix

* delate some wrong message

* 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

* Fix the double grad bug for the star gan. (#25655)

* update the retain_graph parameter doc. test=develop

* Saving inference model for user defined quantization

* Saving inference model for user defined quantization

* 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

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

* 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

* add DistributedSampler and ColorJitter, test=develop

* test=release/1.8, add index sample

test=release/1.8, test=document_fix