Fix configs in num_classes (#2900)

PaddleCV/PaddleDetection/configs/faster_rcnn_x101_vd_64x4d_fpn_2x.yml

@@ -10,6 +10,7 @@ save_dir: output
 pretrain_weights: https://paddle-imagenet-models-name.bj.bcebos.com/ResNeXt101_vd_64x4d_pretrained.tar
 weights: output/faster_rcnn_x101_vd_64x4d_fpn_1x/model_final
 metric: COCO
+num_classes: 81
 
 FasterRCNN:
   backbone: ResNeXt
@@ -76,7 +77,6 @@ BBoxAssigner:
   bg_thresh_lo: 0.0
   fg_fraction: 0.25
   fg_thresh: 0.5
-  num_classes: 81
 
 BBoxHead:
   head: TwoFCHead
@@ -84,7 +84,6 @@ BBoxHead:
     keep_top_k: 100
     nms_threshold: 0.5
     score_threshold: 0.05
-  num_classes: 81
 
 TwoFCHead:
   num_chan: 1024