Tensor2tensor

* New [`RTransformer`](https://github.com/tensorflow/tensor2tensor/blob/master/tensor2tensor/models/research/r_transformer.py) model, a recurrent Transformer
* New [English-Estonian translation dataset](https://github.com/tensorflow/tensor2tensor/blob/master/tensor2tensor/data_generators/translate_enet.py) thanks to stefan-it
* New `ROC_AUC` metric thanks to jjtan
* Various fixes, improvements, additions, etc.

* Multistep Adam Optimizer many thanks to AgoloCuongHoang for contributing in 1773 !
* Residual Shuffle-Exchange Network thanks to EmilsOzolins in 1805 !
* Not pinning the gym version.

Added basic support for TF2 modeling in f65b5e4e0be50b284f9b21d56d3d2a46792cdecf thanks to rjpower !

Other misc fixes:
- Fixing feature encoder for tf.string variable length features.
- adding hparam to make encoder self-attention optional.
- Documentation update, thanks w-hat

PRs:
* 1788 by mzilinec adding an option to select different TPU zone.

Some more code cleanups, regarding tf.compat.v1

* Flush out some more contrib remnants.

* Some changes to handle 1.x to 2.x for tf contrib
* TODO(afrozm): Write more