[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.