Separate API Models from SDK Package

[kramaranya]

We need to move the generated Python models from sdk/python/kubeflow/katib/models to a new directory api/python_api/kubeflow_katib_api/ and set up proper Python package. The OpenAPI generator script will need to be adjusted too. Similar to kubeflow/trainer#2632

We are migrating Katib SDK to Kubeflow SDK, but the generated models should remain in the Katib repo. This will eventually allow us to publish the models to PyPI, which SDK will import then.

cc @kubeflow/kubeflow-sdk-team

[kramaranya]

/assign