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)

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[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))