Merge pull request !4632 from ZiruiWu/map_callback

implment pause in MapOp, added more to callback

add ds_callback

- Initial drop of Python DSCallback

- Pybind DSCallback

- Pybind DSCallback

added callback to mapOp

- de_pipeline DSCallback

- de_pipeline DSCallback

add test case, segfault for now

fix seg fault

- de_pipeline DSCallback

remove 1 line

update callback test case, now works

use builder class for mapOp callback

- de_pipeline DSCallback

- de_pipeline DSCallback

- de_pipeline DSCallback

better test case

minor fix

add comments and minor clean ups

get rid of nullptr in MapOp, use other flag instead

fix a bug ParseMapOp only takes 1 callback

- Added WaitedDSCalabck

refactor callback param

fix text case incorrect number

- added testing

fix cpp test case

- added testing

- revert back lenet changes

- cleanup test_callbacks.py

- cleanup test_callbacks.py

fix CI stage I

fix CI stage II

fix CI and update epoch counter

- add validation
- add more testing  test_callbacks.py

use random data op to do tests

adjust when to call EpochBegin/End

- add repeat with callback

- addressing reviewers' comments

- docstring and CI fixes

- docstring and CI fixes

- docstring and CI fixes

- rebase with upstream/master

fix cpp test case

fix review comments

addr review cmts, add test case

Merge pull request !4597 from luoyang/c-api

Merge pull request !4755 from liuzhongkai/memory_leak1

Merge pull request !4703 from tom_chen/deepfm_gpu

Merge pull request !4749 from zhaodezan/master

Merge pull request !4759 from jiangzhiwen/dataset/fix_duplicate_column_names

Merge pull request !4616 from ling/deconv

Merge pull request !4756 from wangshaocong/lite_clean

Merge pull request !4685 from ZPaC/master-supports-export-server-ckpt

Merge pull request !4753 from ghzl/fix-quant-param-null

Merge pull request !4750 from 陶云浩/lite

Merge pull request !4704 from chengang/fix_ps_multi_worker

fixed pylint errors

Merge pull request !4727 from chenzupeng/master-lite

Merge pull request !4740 from guozhijian/fix_api_comment

Merge pull request !4751 from zongha/master

Merge pull request !4651 from 徐安越/master

Merge pull request !4649 from yepei6/comment

Merge pull request !4721 from fuzhiye/tmp

Merge pull request !4725 from chenjianping/lite_dev2

Merge pull request !4736 from huangxinjing/wide-deep-mutli-table-pos

Merge pull request !4569 from hanjun996/master

Merge pull request !4696 from jiangzhiwen/jzw/c_api_csv

2.rewrite resize func

Merge pull request !4693 from wangshaocong/lite_cmake

Merge pull request !4739 from lixian/master

Merge pull request !4709 from wangzhe/master

Merge pull request !4555 from wangshaocong/lite