Tensor2tensor

* Lambada and wikitext103 datasets.
* ASR model with Transformer and iPython notebook.
* Many other improvements including RL code, autoencoders, the latent transformer (transformer_vae) and more.

* `--problems` command-line flag renamed to `--problem`
* `hparams.problems` renamed to `hparams.problem_hparams` and `hparams.problem_instances` renamed to `hparams.problem` (and neither are lists now)
* Dropped support for TensorFlow 1.4
* Various additions, fixes, etc.

* Distillation codepath added
* Improved support for serving language models
* New `TransformerScorer` model which return log prob of targets on `infer`
* Support for `bfloat16` weights and activations on TPU
* SRU gate added to `common_layers`
* `--checkpoint_path` supported in interactive decoding
* Improved support for multiple outputs
* `VideoProblem` base class
* Various fixes, additions, etc.

* Scalar summary support on TPUs
* New `Squad` and `SquadConcat` problem for question answering (and relevant base class)
* New video problems
* `bfloat16` support for `Transformer` on TPUs
* New `SigmoidClassLabelModality` for binary classification
* Support batch prediction with Cloud ML Engine
* Various fixes, improvements, additions

* Updates to experimental RL codebase
* `ImageTransformer` on TPU
* Various updates, fixes, additions, etc.