Performance issues in src/data/dataset.py(P2)

[DLPerf]

Hello,I found a performance issue in the definition of _get_training_data , artem-oppermann_Deep-Autoencoders-For-Collaborative-Filtering/src/data/dataset.py, dataset = dataset.map(parse) 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 dataset2 = dataset2.map(parse) , dataset = dataset.map(parse),

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.