Tensor2tensor

Some changes needed to be able to import problems with TF 2.0

* Move away from tf.flags to absl-py's flags.
* Move away from std::string to tensorflow::string

Final T2T major release
It is now in maintenance mode — we keep it running and welcome bug-fixes, but encourage users to use the successor library [Trax](https://github.com/google/trax).

PRs Merged
- 1724 by Separius - use batch_size in _test_img2img_transformer thanks!
- 1726 by senarvi - Fix decoding in prepend mode thanks!
- 1733 by prasastoadi - En-Id untokenized parallel corpora thanks!
- 1748 by gabegrand adding a Text2RealProblem class -- thanks a lot gabegrand

Bug Fixes
- Fix features and decoding on TPUs by mts42000
- iansimon and Kristy Choi around shape assertions and modalities
- superbobry fixed cases where tf.TensorShape was constructed with float dimensions

Misc
- Trax was moved into its own repo: https://github.com/google/trax

PRs Merged
* 1720 thanks przemb
* 1698 1699 test/util file fixes thanks to Vooblin
* Fix serving response from Cloud ML Engine (1688) thanks to evalphobia
* Refine automatic mixed precision support via hyper param (1681) thanks vinhngx
* correct return shape of rel_pos2abs_pos() (1686) thanks to Separius
* save attention weights for relative attention v2 (1682) thanks to Ghostvv
* Update generator_utils.py (1674) thanks to TanguyUrvoy

Docs
* Transformer tutorial (1675) many thanks to Styleoshin

Problems
* 4 new dialog problems by ricsinaruto in 1642

Models
* Extend NeuralStack to support Dequeu by reading/writing in both directions, thanks narphorium

TRAX
* Lots of work on SimPLe tuning hyperparameters by koz4k , lukaszkaiser and afrozenator
* async data collection for RL in TRAX
* New memory efficient Transformer using Reversible layers, thanks to Nikita Kitaev, lukaszkaiser and Anselm Levskaya
* Losses and metrics are layers now in trax, thanks to lukaszkaiser
* Activations in TRAX thanks to joaogui1 in 1684 and 1666

Models / Layers:
* NeuralStack and NeuralQueue added, in https://github.com/tensorflow/tensor2tensor/commit/838aca4960f851cd759307481ea904038c1a1ab5 - thanks narphorium !
* Open Sourcing the Search Space used in EvolvedTransformer - https://github.com/tensorflow/tensor2tensor/commit/4ce366131ce69d1005f035e14677609f7dfdb580
* Masked local n-D attention added in - https://github.com/tensorflow/tensor2tensor/commit/2da59d24eb9367cbed20c98df559beccd11b7582

Problems:
* Add English-Spanish translation problem (1626) thanks voluntadpear !
* MovingMNist added in https://github.com/tensorflow/tensor2tensor/commit/121ee60a3b57a092264aa5b5bf69ad194cafb118 thanks MechCoder !

Bug Fixes:
* Loss twice multiplied with loss_coef (1627) by davidmrau - thanks a lot David!
* Fix log_prob accumulation during decoding, thanks lmthang !
* Fixed high usage of TPU HBM "Arguments" during serving
in https://github.com/tensorflow/tensor2tensor/commit/d38f3435ded822e585d1fc7136f3ece857a41c8d thanks ziy !
* Should not generate summary during decoding in dot_product_relative_atention (1618) thanks phamthuonghai !

Misc changes:
* Implement sequence packing as a tf.data.Dataset transformation - https://github.com/tensorflow/tensor2tensor/commit/560c008f7d87502174765fac5ae3d822bbf6b243 thanks robieta !
* Lots of work on t2t_distill and model exporting by ziy - thanks ziy !

RL:
Introduce Rainbow. (1607) by konradczechowski in 1607
Changes to MBRL by konradczechowski , koz4k in multiple PRs.

PRs:
* Adding automatic mixed precision support (1637) thanks a lot to vinhngx !
* Documentation for creating own model 1589 thanks hbrylkowski !
* Adding extra linear to semantic hashing discretization bottleneck. 1578 thanks martiansideofthemoon !
* Using partial targets at inference time. (1596) thanks EugKar !
* Updated link to DeepMind Math dataset (1583) thanks MaxSobolMark !
* Only strip end of line (1577) thanks funtion !
* correct typo in add_timing_signal_nd (1651) many thanks to Separius !
* fix decode bug (1645) many thanks to dong-s !
* Change confusing function name (1669) thanks lazylife7157 !

TRAX:

Base
* Forked optimizers from JAX and make them objects in https://github.com/tensorflow/tensor2tensor/commit/1c7c10c60abc31308b40ae6c850e5c9e363dd4a9
* Trax layers are now stateful and support custom gradients.
* Multi-device capability added.
* Memory efficient trainer added in https://github.com/tensorflow/tensor2tensor/commit/b2615aab938af99418ac0d1318338bf3030357fa ! Thanks Nikita Kitaev!
* Adafactor optimizer added in TRAX - https://github.com/tensorflow/tensor2tensor/commit/63c015f964c1166d181d8efd232abd856574fd83
* Demo Colab added in https://github.com/tensorflow/tensor2tensor/commit/cec26dbd782ea7e4c07377e8d1f9391eb0c5a65c thanks levskaya
* Demo colab for trax layers - https://github.com/tensorflow/tensor2tensor/commit/7632ed01e739cd124c8bac85f121f0f49ddd86cf
* Transformer, TransformerLM, [Reversible Transformer](https://github.com/tensorflow/tensor2tensor/commit/8c23cbb2f3634d7ba2d9ade1c88b935e07197218), PositionLookupTransformer and Resnet50 are some of the models that TRAX now supports.

RL
* Many PPO changes to be able to work on Atari.
* Distributed PPO where the envs can run in multiple parallel machines using gRPC
* SimulatedEnvProblem by koz4k - a gym env that simulates a step taken by a trainer of a Neural Network in https://github.com/tensorflow/tensor2tensor/commit/2c761783a7aacd6800d445d10ad3676a56365514
* Implement SerializedSequenceSimulatedEnvProblem
by koz4k
- https://github.com/tensorflow/tensor2tensor/commit/f7f8549a6421723154b366996b2c6559048ac3fb
* Transformer can be used as a policy now, thanks to koz4k in https://github.com/tensorflow/tensor2tensor/commit/33783fd63bd0debe2138c5569698b31d9af350f6 !

Minor fix to 1.13.3, please see release notes there.