fix no label training in bs2 (#3891)

ppdet/modeling/proposal_generator/target.py

@@ -215,7 +215,8 @@ def generate_proposal_target(rpn_rois,
         if gt_bbox.shape[0] > 0:
             sampled_bbox = paddle.gather(gt_bbox, sampled_gt_ind)
         else:
-            sampled_bbox = paddle.zeros([0, 4], dtype='float32')
+            num = rois_per_image.shape[0]
+            sampled_bbox = paddle.zeros([num, 4], dtype='float32')
 
         rois_per_image.stop_gradient = True
         sampled_gt_ind.stop_gradient = True