reuse RetinaNet pipeline for now

* add PPYOLO
Co-authored-by: Nlongxiang <longxiang@baidu.com>

* add py_func check in export_model

* refine error message

* fix with_background in export model

* fix coco_eval background

* optimize python path

* 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

* support face detection in python inference

* support face detection in python deploy

* update cpp_infer for paddle latest

* polish code

* export config file in export_model.py

* add custom label_list & dump_box

* add infer_cfg.yml & doc

* export label_list to inference config

* update PaddleTensor to ZeroCopyTensor

* load inference

* add doc

* add config file for tensorrt_infer

* add padding for rcnn preprocess

* refine code & doc

* 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.

* add export_model doc