Prompt2Prompt: Inherit from DiffusionPipeline

[nxbringr]

What does this PR do?

• Refactors Prompt2PromptPipeline class to inherit from DiffusionPipeline instead of StableDiffusionPipeline class.
• Adds (copies) dependencies from pipeline_stable_diffusion
• Tested it against a few examples mentioned in community README here.

Fixes #6984

Before submitting

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

Who can review?

Apologies for the delay. Looking forward for your review :)
@yiyixuxu

[sayakpaul]

Very nice changes. Thank you!

@yiyixuxu good example where we should check if the "# Copied from ... statements" are taking effect.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[yiyixuxu]

thank you:)