* Clean code
* rm py_op/post_processing.py and utils/data_structure.py

* fix dygraph

* refine keep_ratio in ResizeOp

* update dygraph export model

* delete get_export

* adapt faster and cascade

* add cascade mask

* add cascade rcnn

* update code

* remove comments

* refine resume process and modify code to support eval while batch_size is larger than 1

* modify code according to review

* add export_model and deploy

* fix travis-ci

* update CMakeList & linux doc

* update by comments

* split yolov3 loss using paddle op

* add sync_bn and modify list to LayerList to use sync_bn, add missing op, support cutmix op in dataset, modify reader to use new ops

* modify code according to review

* modify code to run eval.py successfully

* modify code to run eval and train successfully

* modify code to use mixup

* rebase code on lastest dygraph

* modify code to run in sync_bn mode

* modify code according to review

* modify target size of ResizeOp

* update reader in dygraph

* update dataloader some detail

* fix num_classes with background

* split config into models, optimizers, dataset, readers，runtime

* modify code to be suited for launch in 2.0rc

* modify code according to review

* migrate detection ops from Paddle to PaddleDetection

* add unittest for detection ops

* update api & unittest

* add copyright & api name to migrate

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refine PaddleDetection benchmard print, test=develop

* refactor dygraph & add mask rcnn fpn

* fix initialization & minor update

* refine architecture of mask_rcnn

add train&eval 
add reader padding 
add new config 

clear directories & files for dygraph branch 

Add image for visualdl board.

* Update docs
* Update README.md

* fix with_background in export model

* fix coco_eval background

* optimize python path

* add yolov4

* add serving export model
* add paddle serving client in requirement.txt
* remove redundant code with codestyle check
* fix export model doc
* fix according to professional reviews
* move paddle serving model export tutorial to the last of doc

* update check config

* fix slim/xx

* Revert back to `fluid.clip` for gradient clipping

current API is in flux, wait for it to stabilize

* Please CI

still buggy