RAE+DiT support for published checkpoints

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Is your feature request related to a problem? Please describe.
diffusers has AutoencoderRAE support via #13046. There is still no native latent generator path for the published RAE checkpoints. I've been following the research and was excited to see the RAE support; this comes next and would be exciting to have!

Already, the official RAE repo ships a custom Stage-2 DiT-like latent generator and public checkpoints/configs, but today these are not available through a native diffusers model + pipeline + checkpoint conversion flow.

Describe the solution you'd like.
In two stages, probably: (a) inference support for the checkpoints (b) training support.
I have some code ready that I've been able to use to get the checkpoints workable, could turn it around into a PR.

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@sayakpaul @kashif if y'all prefer, I could spin out a PR with some codexing on my current codebase for part (a).