Migrate to TF 2.0/tf.keras

[minimaxir]

I had made textgenrnn with external Keras since native TF was missing features. Now that there is parity, I am OK with merging it back into native TF with TF 2.0 support. textgenrnn does not use much custom Keras code so it should be a relatively simply change; the concern is not breaking old models, which may be possible due to the SavedModel change.

TF 2.1 also has TPU/mixed precision support which will be very helpful for training performance.

[minimaxir]

Handling work in TF-2.0 branch.

[minimaxir]

dammit I forgot about the AttentionWeightedAverage layer. Keras now has a native Attention layer but I suspect they are not compatible.

[ZerxXxes]

Would it be hard to add support for the MultiWorkerMirroredStrategy so that I can train on multiple workers at the same time?

[walter-grace]

The google colab needs updated to TF 2.x as well ;
https://drive.google.com/file/d/1mMKGnVxirJnqDViH7BDJxFqWrsXlPSoK/view

[minimaxir]

Done in 2.0