Disable weights / neff separation of SDXL's UNET for neuron sdk 2.18

[JingyaHuang]

What does this PR do?

Issue reported here: https://github.com/aws-neuron/aws-neuron-sdk/issues/859

Force inline_weights_to_neff=True for sdxl's unet for now to be able to bump optimum-neuron to neuron SDK 2.18, to remove after it's patched by the Annapurna team.

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• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

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