[Dy2stat] Decrease test_yolov3 GPU usage (#24955)

[Dy2stat] decrease the batch size to decrease GPU usage.

python/paddle/fluid/tests/unittests/dygraph_to_static/yolov3.py

@@ -90,7 +90,7 @@ cfg.ignore_thresh = .7
 # SOLVER options
 #
 # batch size
-cfg.batch_size = 8 if fluid.is_compiled_with_cuda() else 4
+cfg.batch_size = 4
 # derived learning rate the to get the final learning rate.
 cfg.learning_rate = 0.001
 # maximum number of iterations