* add save best model & fix dcn configs

* fix yolov3 voc eval in train

* fix comment

* standard SENet and add a warning in coco loader

* add deformable models
* fix dcn v2
* fix log_iter

* fix cn docs

* fix easy problem

* update GETTING_STARTED.md

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

* Fix ResizeImage and data transform for SSD
* Fix code style

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

* add log iter argument

* ppdet log time smooth

* ppdet log eta

* 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

* fix save_inference_model cannot find feed vars

* remove comment

* fix format

* add comment for pruned var

* add log for prune

* fix resnet for resnet34

* fix ResNet18 and ResNet34

* update modelzoo and fix configs

* fix configs

* fix resnet bn