Make the VAE tile size configurable for tiled VAE

[RyanJDick]

Summary

• This PR exposes a tile_size field on ImageToLatentsInvocation and LatentsToImageInvocation.
  • Setting tile_size = 0 preserves the default behaviour.
• This feature is primarily intended to support upscaling workflows that require VAE encoding/decoding high resolution images. In the future, we may want to expose the tile size as a global application config, but that's a separate conversation.
• As a general rule, larger tile sizes produce better results at the cost of higher memory usage.

Example:

Original (5472x5472)

VAE roundtrip with 512x512 tiles (note the discoloration)

VAE roundtrip with 1024x1024 tiles (some discoloration still present, but less severe than at 512x512)

Related Issues / Discussions

Related: #6144

QA Instructions

• Test image generation via the Linear tab
• Test VAE roundtrip with tiling disabled
• Test VAE roundtrip with tiling and tile_size = 0
• Test VAE roundtrip with tiling and tile_size > 0

Merge Plan

No special instructions.

Checklist

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