Performance issues in the program

[DLPerf]

Hello,I found a performance issue in the definition of input_fn , cs230-stanford/cs230-code-examples/blob/master/tensorflow/vision/model/input_fn.py, dataset = (tf.data.Dataset.map was called without num_parallel_calls. I think it will increase the efficiency of your program if you add this.

Here is the documemtation of tensorflow to support this thing.

Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.