[fix] fix num_classes bug in faster rcnn config (#4088)

`num_classes` in model should be the same as the number of dataset class

configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco-person-bicycle-car.py

 _base_ = './faster_rcnn_r50_fpn_1x_coco.py'
+model = dict(roi_head=dict(bbox_head=dict(num_classes=3)))
 classes = ('person', 'bicycle', 'car')
 data = dict(
     train=dict(classes=classes),

configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco-person.py

 _base_ = './faster_rcnn_r50_fpn_1x_coco.py'
+model = dict(roi_head=dict(bbox_head=dict(num_classes=1)))
 classes = ('person', )
 data = dict(
     train=dict(classes=classes),