* fix obj365 cascade rcnn config
* fix obj365 cascade rcnn doc

* fix dead link in ppdet/slim

* add distill_pruned_model demo, python scripts and README

* exit1 in travis failed

* fix prune.py

* update YOLOv3_ENHANCEMENT.md
* update IoUloss and DropBlock configs for YOLOv3 related configs

* add slim MODEL_ZOO to docs

* add prune/eval.py and update model zoo

* fix all documentation

* fix docs conf.py

* fix some error

* fix readthedocs

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

* add iou loss to yolov3
* modify the comment and delete redundant yml file
* fix the low speed bug in dropblock module

* add slim model zoo

* update merge api
* add coco distillation demo
* fix details
* resolve some issues

* Add latency table
* Add latency demo
* Update doc

* add slim quantization demo

* add quantization demo

* add skip quant

* fix details in doc

* fix details

* fix details

* fix details

* fix details

* fix details

* fix details

* add model zoo link in doc

* remove result in doc

* add dropblock module
* move dropblock module from yolo_head to ops
* fix obj365 num classes

* add blazeface nas demo

* fix format

* fix configs

* fix reduce_rate

* add gcnet model : https://arxiv.org/abs/1904.11492

* refine code & doc for cpp_infer

* add cython for pycocotools install

* add diou nms.
* fix typo in diou nms and soft nms (lod_leval->lod_level).

* update distillation demo

* load inference

* add doc

* add config file for tensorrt_infer

* add padding for rcnn preprocess

* refine code & doc

* refine cocoapi import

* fix infer when anno_path is None

`--minimal` is option for `generate` and should be positioned after it

* Add GIOU loss.
* Add DIOU/CIOU loss.
* Change loss box type in bbox_head.py.
* Remove fuse_elewise_add_act_ops flag in train.py (Error may happen for elem_divbased on the flag).