* fix train on VOC on multi-GPU hang

* not set persistable in parse_fetches

* add gn doc

# Add VGG-SSD on VOC and COCO dataset.
# Add config and model zoo. 
# Refine bbox2out and draw_bbox:
    - Add bbox de-normalize in bbox2out function.
    - Remove bbox de-normalize in draw_bbox.

* refine weight download

* fix comment

* add save best model & fix dcn configs

* fix yolov3 voc eval in train

* fix comment

* fix easy problem

Add PaddleDetection docs & some model configs

* add voc_eval and yolo_darknet_voc

* add yolov3_darknet_voc in MODEL_ZOO

* fix default im_size

* fix MODEL_ZOO note

* fix is_bbox_normalized

* extract map to map_utils.py

* update yolov3_dorknet_voc mixup

* add yolov3_r34_voc

* add yolov3_mobilenet_v1_voc

* fix drop empty in VAL mode

* use cfg.num_classes

* assert metric valid

* enable difficulty can be None

* add comment for bbox_eval

* num_classes in retinanet

option should not be overridden by subsequent options of the same module.

* fix infer doc and config default

* refine dataset exists

* fix None proc

* add rpn recall function and class-aware rpn

* optimize error message when result is none

* check gpu for ppdet/yolov3

* add random shape descp in MODEL_ZOO

* refine doc

* not use core

* Rename object_detection to PaddleDetection
* Small fix for doc