Make Batch Size Smaller on Mac Because of CI Machine (#28569)

I found the unittest failed due to batch size. Maybe the reason is that our CI machine has limited memory. I decreased the batch size.

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

@@ -15,6 +15,9 @@
 from __future__ import division
 from __future__ import print_function
 
+import os
+import sys
+
 import paddle.fluid as fluid
 from paddle.fluid.dygraph import declarative
 from paddle.fluid.dygraph.base import to_variable
@@ -90,7 +93,7 @@ cfg.ignore_thresh = .7
 # SOLVER options
 #
 # batch size
-cfg.batch_size = 4
+cfg.batch_size = 2 if sys.platform == 'darwin' or os.name == 'nt' else 4 
 # derived learning rate the to get the final learning rate.
 cfg.learning_rate = 0.001
 # maximum number of iterations