Created by: heavengate

1. add save_inference_model example for yolov3 User can save_inference_model in yolov3 eval.py/infer.py by uncomment following code

# you can save inference model by following code
# fluid.io.save_inference_model("./output/yolov3", 
#                               feeded_var_names=['image', 'im_shape'],
#                               target_vars=outputs,
#                               executor=exe)

2. refine link in README.md and README_en.md to relative link e.g. https://github.com/PaddlePaddle/models/blob/623698ef30cc2f7879e47621678292254d6af51e/PaddleCV/yolov3/reader.py#L39 -> ./reader.py#L39