Add snippets to KerasNLP models

[Wauplin]

In #616 we added basic support for the keras-nlp library. This PR adds a code snippet for KerasNLP models. I just got confirmation from @mattdangerw that models can be loaded with:

import keras_nlp

tokenizer = keras_nlp.models.Tokenizer.from_preset("hf://${model.id}")
backbone = keras_nlp.models.Backbone.from_preset("hf://${model.id}")

We might want to do more fancy stuff in the future to parse the underlying model architecture server-side but in the meantime this is a valid solution.

[julien-c]

lgtm!

note that parsing metadata.json or config.json to automagically set tags or snippets is valid too (instead of, or in complement to, storing stuff in the model card)

[osanseviero]

LGTM, thanks!

[Wauplin]

Thanks for the reviews!