Performance issues in the program

[DLPerf]

Hello,I found a performance issue in the definition of dataset ,
omoindrot/tensorflow-triplet-loss/blob/master/model/mnist_dataset.py,
images = images.map(decode_image) 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 labels = tf.data.FixedLengthRecordDataset(labels_file, 1, header_bytes=8).map(decode_label)

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.