* Refine and clean code
* Fix conflicts
Co-authored-by: NKaipeng Deng <dengkaipeng@baidu.com>

* add ssd_vgg16_120e_coco

* 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

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

* modify code according to review

* solve load optimizer if some key not in optimizer

* variable name tmp_key to key

* add comment

* rename GridMaskOp to GridMask in ppdet/data/transform/operator.py to avoid conflict

* fix bugs in operator and batch_operator and check_version

* modify name in GridMask

* add comma according to review

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

* refactor dygraph & add mask rcnn fpn

* fix initialization & minor update

* refine architecture of mask_rcnn

add train&eval 
add reader padding 
add new config 

add cascade rcnn and modify make net method
add new cfg method 
add auto debug tool

add faster rcnn\mask\cascade rcnn model arch\module\op
add base debug func in bufferdict 

* add version check

* update version check & python version for cornerpool

* fix with_background in export model

* fix coco_eval background

* add yolov4

* Make logger less noisy

* Fix indent

* update check config

* fix slim/xx

* code for cornernet

* add doc

* refine custom op

* refine code from review comments

* refine doc

* update doc

* update code

* add cornernet_squeeze_mixup_cosine config

* update code

* update code

* add yolov3_mobilenet_v3

* refine transfer learn

* refine transfer learning doc

* Update checkpoint to use paddle 1.7 API

* Add new checkpoint support to `load_and_fusebn`

* Make yapf happy

* exit1 in travis failed

add libra rcnn, including
  * libra loss
  * libra fpn(bfp)
  * libra sampling