TFTS: Make it easier to swap in different autoregressive models.

Adds a very simple LSTM encoder/decoder option as an example.

ARModel's new constructor argument is a bit awkward, since Estimator's new graphs mean we need a Model factory rather than a Model (or to un-build the model?). It's still a much more pleasant way to write autoregressive models than fiddling with ARModel directly, since ARModel handles collecting all the features (and the prediction loop, etc.). Happy to hear other ideas for an API.

PiperOrigin-RevId: 195436186