SSD with GridMask base on lufficc/SSD

Train example

python train.py --config-file configs/vgg_ssd300_voc0712.yaml
python train_grid.py --config-file configs/vgg_ssd300_voc0712.yaml
python train_labels_grid.py --config-file configs/vgg_ssd300_voc0712_labels_grid.yaml

more details, please refer to lufficc/SSD and Jia-Research-Lab/GridMask