fix getting default diffusion pipeline parameters from config

[eaidova]

What does this PR do?

model_index.json may contain overrides of default pipeline parameters e.g.
https://huggingface.co/stabilityai/stable-diffusion-xl-base-1.0/blob/main/model_index.json#L4

now these parameters are ignored at initialization stage of openvino pipeline.

Fixes # (issue)

Before submitting

• 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?

[HuggingFaceDocBuilderDev]

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