[CIs]Refactoring Inference tests & Improve CIs speed

[JingyaHuang]

What does this PR do?

• Refactoring diffusers inference tests
• Increase CI speed

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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[tengomucho]

LGTM!

[dacorvo]

LGTM, thanks ! I think we could go one step further and reduce the number of tests in the amount of legacy models tests, so that the corresponding pipelines are completed in less than 30 minutes