fix yolov3 return value in dygraph mode. test=develop (#40185)

Author: heavengate

python/paddle/fluid/tests/unittests/test_yolov3_loss_op.py

@@ -305,6 +305,7 @@ def test_dygraph(self):
  use_label_smooth=True,
  scale_x_y=1.)
  assert loss is not None
+ assert loss.shape == [2]
  paddle.enable_static()
 
 

python/paddle/vision/ops.py

@@ -195,7 +195,7 @@ def yolo_loss(x,
  """
 
  if in_dygraph_mode() and gt_score is None:
- loss = _C_ops.yolov3_loss(
+ loss, _, _ = _C_ops.yolov3_loss(
  x, gt_box, gt_label, 'anchors', anchors, 'anchor_mask', anchor_mask,
  'class_num', class_num, 'ignore_thresh', ignore_thresh,
  'downsample_ratio', downsample_ratio, 'use_label_smooth',