[Enhancement] Support Prompt-to-prompt, ddim inversion and null-text inversion

[FerryHuang]

Support Prompt-to-prompt, ddim inversion and null-text inversion.

Considering that these three methods share common features and can be combined, I implemented them as modules in this project.

Checklist

Before PR:

• [√] I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• [√] Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• [√] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• [√] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• [√] The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[LeoXing1996]

@FerryHuang Sorry for late response. The implementation is already good enough. However, there is one aspect that needs improvement: directly uploading images will increase the repository's size. Please change the images in the markdown to image hosting links.

For example, you can drag and drop an image into the chat box in Github and a link will be generated. This link will be permanently saved in Github and you can use it in your markdown.

[FerryHuang]

@FerryHuang Sorry for late response. The implementation is already good enough. However, there is one aspect that needs improvement: directly uploading images will increase the repository's size. Please change the images in the markdown to image hosting links.

For example, you can drag and drop an image into the chat box in Github and a link will be generated. This link will be permanently saved in Github and you can use it in your markdown.

Thanks for the trick! And changes have been made~

[LeoXing1996]

LGTM! Great work!