doc of image_classification: lack of definition of predict
Created by: Genie-Liu
before train_loop() , we should add the definition of predict
predict = inference_program()these lines cause error: python NameError: name 'predict' is not defined
https://github.com/PaddlePaddle/book/blob/2a6a637dc9e70d8683536fa2ef4a6d1d74fca8e0/03.image_classification/index.cn.html#L486-L489