Performance issues in the program

[DLPerf]

Hello,I found a performance issue in the definition of parse_files_to_dataset ,
zihangdai/xlnet/blob/master/data_utils.py,
dataset = dataset.cache().map 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.