Reduce batch size ot fix CPU memory, test=develop (PaddlePaddle#29736)

Unit test reported memory not enough on CPU machines. Reduce batch size again.

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

@@ -93,7 +93,7 @@ def __setattr__(self, name, value):
 # SOLVER options
 #
 # batch size
-cfg.batch_size = 2 if sys.platform == 'darwin' or os.name == 'nt' else 4 
+cfg.batch_size = 1 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