Performance issues in tpu/models/official/retinanet/dataloader.py

[DLPerf]

Hello,I found a performance issue in the definition of __call__ ,
tpu/models/official/retinanet/dataloader.py,
dataset = dataset.map(_process_example) 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(parser) ,
dataset = dataset.repeat().map(parser)

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.