Add comments for num_classes in SSD model (#3281)

configs/ssd/ssd_mobilenet_v1_voc.yml

@@ -11,6 +11,7 @@ metric: VOC
 map_type: 11point
 save_dir: output
 weights: output/ssd_mobilenet_v1_voc/model_final/
+# 20(label_class) + 1(background)
 num_classes: 21
 
 SSD:

configs/ssd/ssd_vgg16_300_voc.yml

@@ -12,6 +12,7 @@ map_type: 11point
 pretrain_weights: https://paddle-imagenet-models-name.bj.bcebos.com/VGG16_caffe_pretrained.tar
 save_dir: output
 weights: output/ssd_vgg16_300_voc/model_final/
+# 20(label_class) + 1(background)
 num_classes: 21
 
 SSD:

configs/ssd/ssd_vgg16_512_voc.yml

@@ -12,6 +12,7 @@ map_type: 11point
 pretrain_weights: https://paddle-imagenet-models-name.bj.bcebos.com/VGG16_caffe_pretrained.tar
 save_dir: output
 weights: output/ssd_vgg16_512_voc/model_final/
+# 20(label_class) + 1(background)
 num_classes: 21
 
 SSD: