Added Neural Machine Translation Tutorial

[rish-16]

The tutorial covers tokenization, cleaning, pre-processing, and configuring TPUs using a parallelly-distributed training strategy. The whole tutorial is written in tf.keras.

[tfdocsbot]

Preview and run these notebook edits with Google Colab:

• site/en/tutorials/sequences/neural_machine_translation_on_cloud_tpus.ipynb

Notebook diffs available on ReviewNB.com.

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[rish-16]

@googlebot I signed it!

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[lamberta]

Thanks for the submission!
We're only working on TF2 docs now so this would need some changes before consideration into the official docs: switch to TF2 (eager by default, no Sessions, etc). And, as a tutorial, it needs more explanations before code cells.
After switching to TF2, you may consider submitting it to the community section of the tensorflow/examples repo.