Port tab transformer example to keras core

[anas-rz]

Port tab transformer example to keras core. Tried testing on all backends and the StringLookup layer doesn't work with torch and jax backends. COLAB to reproduce

[fchollet]

Tried testing on all backends and the StringLookup layer doesn't work with torch and jax backends

Other backends don't have a string dtype, so you won't be able to include the StringLookup layer in the model. The solution would be to move string preprocessing outside the model, to the tf.data pipeline.

[anas-rz]

Please consider the latest commit. I accidentally put the same old code in the previous commit.

[fchollet]

LGTM, thank you! Sorry for the delay in reviewing, I had lost track of this.