Add gradient blending to tile seams in MultiDiffusion

[RyanJDick]

Summary

This PR adds gradient blending between tiles during MultiDiffusion denoising. Previously, no gradient was applied and the sharp cut-off caused visible seams in some case (particularly evident on smooth single-color regions).

Before

Upsampled to 2240x1872

Upsampled to 4480x3744

After

Upsampled to 2240x1872

Upsampled to 4480x3744

Other Notes:

• The non-gradient blending was based on the official MultiDiffusion implementation that accompanied the paper - this deviates from that.
• Other ideas that would likely help with this problem:
  • Large overlaps between tiles so that each tile edge is covered by multiple other tiles. (This is what the official MultiDiffusion implementation seems to do.)
  • Jitter the tile locations so that seams aren't in the same place in every step.

Aside: Tile color shift

As seen in the sample images, there is a remaining issue with tile color shift. We currently rely on a ControlNet to maintain this consistency, but clearly it is not very effective. Future improvements that might help with this:

• Larger MultiDiffusion tile sizes and larger overlaps
• Shift latent tile distributions close to original latent tile distributions
• Use noise inversion to guide the process (i.e. like SUPIR, DemoFusion, and others)
• Apply color correction in pixel space on the final image

QA Instructions

Speed tests on RTX4090:

• Denoising during upscaling to 2240x1872:
  • Before: 36s
  • After 36s
• Denoising during upscaling to 4480x3744
  • Before: 150s
  • After: 150s
    I.e. no change in speed on an RTX4090.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)