Fix diffusers import prompt2prompt

[nxbringr]

What does this PR do?

• Fix import statement for diffusers in Prompt2Prompt.
• Update the ReadMe of Prompt2Prompt with the correct example implementation.

Fixes #6842
Documentation change done in examples/community/README.md, Prompt2Prompt example.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• 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?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

• For the documentation change. @yiyixuxu
• For the import statement change @patrickvonplaten and @sayakpaul

[patrickvonplaten]

cc @UmerHA could you also check quickly if you have a minute? :-)

[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.

[nxbringr]

The checks are failing for test_pipelines.py/test_download_only_pytorch with the following error (complete error trace here):

FAILED tests/pipelines/test_pipelines.py::DownloadTests::test_download_only_pytorch - requests.exceptions.ReadTimeout: (ReadTimeoutError("HTTPSConnectionPool(host='huggingface.co', port=443): Read timed out. (read timeout=10)"), '(Request ID: b6b03b3e-ecfe-4669-9642-9c6522fdb5a7)')

This particular test works perfectly fine on my local. Given it's a timeout error, do you think its worth restarting the Fast tests for PRs workflow?

cc: @patrickvonplaten @UmerHA