fix RCNNBox, test=dygraph (#2216)

dygraph/ppdet/modeling/layers.py

@@ -318,7 +318,8 @@ class RCNNBox(object):
 
         origin_shape = paddle.concat(origin_shape_list)
 
-        # [N, C*4]
+        # bbox_pred.shape: [N, C*4]
+        # C=num_classes in faster/mask rcnn(bbox_head), C=1 in cascade rcnn(cascade_head)
         bbox = paddle.concat(roi)
         if bbox.shape[0] == 0:
             bbox = paddle.zeros([0, bbox_pred.shape[1]], dtype='float32')
@@ -326,10 +327,9 @@ class RCNNBox(object):
             bbox = delta2bbox(bbox_pred, bbox, self.prior_box_var)
         scores = cls_prob[:, :-1]
 
-        # [N*C, 4]
-
-        bbox_num_class = bbox.shape[1] // 4
-        bbox = paddle.reshape(bbox, [-1, bbox_num_class, 4])
+        # bbox.shape: [N, C, 4]
+        # bbox.shape[1] must be equal to scores.shape[1]
+        bbox_num_class = bbox.shape[1]
         if bbox_num_class == 1:
             bbox = paddle.tile(bbox, [1, self.num_classes, 1])