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

* add faster_rcnn_r50_fpn and faster_rcnn_r50

* update TestReader in dygraph

* if gt_ploy is empty, continue

* fix typo

* judge gt_ploy is empty or not

* fix judge condition

* fix gt_ploy judge

* 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

* refactor dygraph & add mask rcnn fpn

* fix initialization & minor update

* refine architecture of mask_rcnn

* filter illegal data

* update warning message

* add yolov4

* Make logger less noisy

* Fix indent

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs

* refine cocoapi import

* fix infer when anno_path is None

* Polish Reader to simplify preprocessing logic.
#  sample_transforms-> make batch -> batch_transforms in Reader.
* Clean some code
* Imporve yolov3_r50vd_dcn_obj365_pretrained_coco 41.4 to 41.8.
* Update all configs.