Add public API export statements

[fchollet]

We're switching to a packaging system that uses an allowlist for public API symbols. All public API symbols (classes, functions) should be decorated with @keras_nlp_export as follows:

from keras_nlp.api_export import keras_nlp_export

@keras_nlp_export("keras_nlp.layers.FNetEncoder")
class FNetEncoder(keras.layers.Layer):
    ...

Note that these statements include serialization, so you don't need any register_keras_serializable decorator calls anymore.

For an example PR, see https://github.com/keras-team/keras-nlp/pull/811/files

We need to do this for all symbols listed on keras.io: https://keras.io/api/keras_nlp/

[kanpuriyanawab]

I would like to take up this issue !

[fchollet]

@shivance awesome -- please open a PR (or multiple PRs for different sections of the API...)

[fchollet]

@shivance with the outstanding PRs, we have everything covered (I think?). Thanks a lot for the help with this!

[kanpuriyanawab]

@shivance with the outstanding PRs, we have everything covered (I think?). Thanks a lot for the help with this!

Cheers !