RegionalPrompting: Inherit from Stable Diffusion

[b-sai]

What does this PR do?

This pull request refactors the RegionalPromptingStableDiffusionPipeline class to inherit from the DiffusionPipeline class rather than the StableDiffusionPipeline class

I tested it on the examples here and it is replicating similar images

Fixes issue #6984

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• refactors pipelines inheriting from StableDiffusionPipeline #6984
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• Did you write any new necessary tests?

cc: @yiyixuxu