Performance issues in the definition of `main`,blob/master/sparse_classifier.py

[DLPerf]

Hello,I found a performance issue in the definition of main , tobegit3hub/tensorflow_template_application/blob/master/sparse_classifier.py, train_dataset.map was called without num_parallel_calls. I think it will increase the efficiency of your program if you add this.

The same issues also exist in validation_dataset.map , train_dataset.map and other three places

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.