add batch_size in YOLOv3Loss for backward compatable. (#1468)

* add batch_size in YOLOv3Loss for backward compatable.

ppdet/modeling/losses/yolo_loss.py

@@ -41,8 +41,10 @@ class YOLOv3Loss(object):
     __inject__ = ['iou_loss', 'iou_aware_loss']
     __shared__ = ['use_fine_grained_loss', 'train_batch_size']
 
-    def __init__(self,
+    def __init__(
+            self,
             train_batch_size=8,
+            batch_size=8,  # stub for backward compatable
             ignore_thresh=0.7,
             label_smooth=True,
             use_fine_grained_loss=False,