fix class_num in quickstart

ppcls/configs/quick_start/ResNet50_vd.yaml

@@ -4,7 +4,6 @@ Global:
   pretrained_model: null
   output_dir: ./output/
   device: gpu
-  class_num: 102
   save_interval: 1
   eval_during_train: True
   eval_interval: 1
@@ -18,6 +17,7 @@ Global:
 # model architecture
 Arch:
   name: ResNet50_vd 
+  class_num: 102
 
 # loss function config for traing/eval process
 Loss: