Performance issues in 7.%20ResNet/train.py(P2)

[DLPerf]

Hello,I found a performance issue in 7.%20ResNet/train.py ,
train_db = train_db.shuffle(50000).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 test_db = test_db.map ,
db = db.shuffle(1000).map,
db = db.shuffle(1000).map,

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.