Performance issues in blob/master/dl/tensorflow/tests/1.13/dataset.py(P2)

[DLPerf]

Hello,I found a performance issue in the definition of dataset ,
dockerfiles/blob/master/dl/tensorflow/tests/1.13/dataset.py,
.map(decode_image) 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 .map(decode_label) ,
.map(decode_image),
.map(decode_label),

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.