* 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

* add matrix nms api

* add test error in matrix nms ut

* modify code according to review

* modify matrix nms api

* add yolo_box and add unittest of yolo_box.

* update comment

* update sample code, call op in ppdet

* migrate roi_align and roi_pool to ops

* 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

clear directories & files for dygraph branch 

* optimize python path

* 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.

* Rename object_detection to PaddleDetection
* Small fix for doc