Add StableDiffusion3PAGImg2Img Pipeline + Fix SD3 Unconditional PAG

[painebenjamin]

What does this PR do?

This PR does two things:

1. Adds StableDiffusion3PAGImg2ImgPipeline, tests and documentation.
2. Fixes a bug with SD3 + PAG in general which caused unconditional PAG generation to fail (PAGJointAttentionProcessor2_0 was failing due to it receiving attention_mask as a keyword argument.)

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 documentation guidelines, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone who is interested, but particularly @yiyixuxu and @asomoza

Images

All are with SD3-Medium-Diffusers

Test output - PAG+CFG

Test output - PAG only

Docstring example output (PAG+CFG):

[jeongiin]

Suggested change

	self._pag_adaptive_scale = pag_adaptive_scale #
	self._pag_adaptive_scale = pag_adaptive_scale

[jeongiin]

Hi! I found this work interesting while reading it and noticed what seemed to be a typo, so I removed it.