Performance issues in utils/dataset_util.py

[DLPerf]

Hello,I found a performance issue in the definition of read_dataset ,
utils/dataset_util.py,
dataset = dataset.interleave 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.