Performance issues in train_nets.py(P2)

[DLPerf]

Hello,I found a performance issue in train_nets.py ,
dataset = dataset.map(parse_function) 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 dataset = dataset.map(parse_function)

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.