"Distributed Training with Keras" (keras.ipynb) Transformer reference is 404

At the end of the notebook there are two references pointing inside the tensorflow Github repo: a [Transformer](https://github.com/tensorflow/models/blob/master/official/nlp/transformer/transformer_main.py) and an [NCF](https://github.com/tensorflow/models/blob/master/official/recommendation/ncf_keras_main.py). The [NCF](https://github.com/tensorflow/models/blob/master/official/recommendation/ncf_keras_main.py) still stands today, but the transformer was moved to a legacy section.

We could use repo permalinks, or this time I've found the ref in the legacy folder: https://github.com/tensorflow/models/blob/master/official/legacy/transformer/transformer_main.py