Performance issues in the program

[DLPerf]

Hello,I found a performance issue in the definition of create_tensorrec_dataset_from_tfrecord , jfkirk/tensorrec/blob/master/tensorrec/input_utils.py, tf.data.TFRecordDataset(tfrecord_path).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.