Performance issues in d2l/tensorflow.py(P2)

[DLPerf]

Hello,I found a performance issue in the definition of load_data_fashion_mnist ,
d2l/tensorflow.py,
.map(resize_fn)) 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.