* use yapf to format all python file

* yapf exclude two unittests file for they rely on writing and reading file, and format will break them

* disable diff_py_file because too many diff files cause command following failed

* alias yolo_loss & decode_yolo_box to paddle.vision. test=develop

* add scale_x_y for yolo_box, test=develop

* refine eps in iou_similarity, test=develop

* polish the label_smooth

test=develop

* polish code

test=develop