Flux latents fix

[DN6]

What does this PR do?

This PR: #9711 changed the latent preparation step so that the vae_scale_factor and default sample size are more clear.

However, we overlooked the fact that this operation (with scale_factor == 16)

        height = 2 * (int(height) // self.vae_scale_factor)
        width = 2 * (int(width) // self.vae_scale_factor)

Is not equivalent to this operation with scale_factor == 8

height = int(height) // self.vae_scale_factor

For resolutions that are divisible by 8 but not by 16 the current pipeline on main will error out, because the implementation does not account for the fact that the latent width and height must be divisible by 2 for the packing step to work.

The old implementation scaled the height and width by //16 and then multiplied by 2. This results in slightly resized images if the height or width aren't multiples of 16.

This PR:

1. Adds steps to account for latent packing. Which will result in resized output images (like we had before)
2. Adds a warning that the image will be resized if an incompatible image is provided
3. Adds fast tests to check Flux with expected and unexpected image shapes to verify that the scaling is properly applied.

Fixes # (issue)

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 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.

[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.

[bghira]

this seems really important but considering how slowly things move around here, not so sure it will be merged in a timely manner.

[yiyixuxu]

oh thanks for the fix!
I made a comment

[yiyixuxu]

is this same as this? (basically revert back to the original code but divisible by self.vae_scale_factor*2
a little bit easier to understand, no?

height = 2 * (int(height) // (self.vae_scale_factor*2))

[DN6]

Failing LoRA tests are unrelated. Merging.