Keras v3 Support

[calad0i]

A# Description

Add keras v3 specific object-based parser and some layer handlers (no h5 or json loading supported). The current keras parser doesn't work with v3 functional models in general.

Type of change

• New feature (non-breaking change which adds functionality)

Tests

test/pytest/test_keras_v3_api.py

Test Configuration:

Requires keras>=3.0. Skips the whole module if this requirement is not sufficed.

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.

[calad0i]

This PR will be rebased after its prerequisites are merged to resolve the conflicts.

[jmitrevs]

I don't know if the case was used in practice, but the old logic allows model to be a dict, and only if it's not not a dict, only then model_arch = json.loads(model.to_json()). Does the logic here work if model is a dict?

[vloncar]

dict is a representation of a json. we work with a dict that was a json of a model whether made by us or, in the old days, by the user.

[calad0i]

model.get_config has slightly different behavior, where objects may be passed as-is (not serialized). v2 converter assumes a dict of fully serialized model config, so serialize+deserialize is used here.