fix compile_only mode for diffusers with transformer as main model

[eaidova]

What does this PR do?

compile_only=True failed for flux/sd3 models

raise ValueError("`compile_only` expect that already compiled model will be provided")

due to checking the model that does not exists in pipeline

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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[echarlaix]

LGTM, thanks @eaidova