从config.Config中获取了默认参数：
C = config.Config()
从get_data中获取了图片、类别计数、类别映射:
all_imgs, classes_count, class_mapping = get_data()
C.class_mapping = class_mapping
从all_imgs中获取训练集、验证集和测试集的图片：
train_imgs = [s for s in all_imgs if s['imageset'] == 'trainval']
val_imgs = [s for s in all_imgs if s['imageset'] == 'test']
