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

* support face detection in python inference

* support face detection in python deploy

* add devices_num on reader

* refine capacity value

* refine bufsize

* 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

* Update README.md
* Update change logs
* Update docs/advanced_tutorials/inference/INFERENCE.md

* add padstride in cpp_infer and refine doc
* refine doc for export_model

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

* add blazeface nas demo

* fix format

* fix configs

* fix reduce_rate

* refine code & doc for cpp_infer

* add cython for pycocotools install

* load inference

* add doc

* add config file for tensorrt_infer

* add padding for rcnn preprocess

* refine code & doc