Add a `TiledStableDiffusionRefineInvocation` (for upscaling workflows)

[RyanJDick]

Summary

Adds a TiledStableDiffusionRefineInvocation node with the following characteristics:

• The node applies the following sequence:
  • Split tiles in image space
  • VAE encode
  • Denoise
  • VAE decode
  • Merge tiles in image space
• The same process can be achieved in a workflow with existing nodes. The primary reason for creating this standalone node is to avoid the need for an 'iterate' node to achieve this workflow. Iterate nodes have some limitations, and are not currently permitted in the hosted InvokeAI app.
• The TiledMultiDiffusionDenoiseLatents node serves a similar purpose in upscaling workflows. Here are the rough tradeoffs between them:
  • MultiDiffusion upscaling requires using a tiled VAE, which can cause some discoloration issues
  • MultiDiffusion is less prone to visible seams.
  • TODO: Speed comparison

TODO: Add comparison images.

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Merge Plan

Checklist

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