Add PRXPixelPipeline: pixel-space PRX text-to-image pipeline

[DavidBert]

What does this PR do?

Adds a pixel-space variant of PRX that denoises raw RGB directly (no VAE), conditioned on a Qwen3-VL text encoder:

• PRXTransformer2DModel: new optional config args bottleneck_size (two-layer img_in projection for large patch dims) and resolution_embeds (PRXResolutionEmbedder conditions the timestep modulation on the latent resolution)
• PRXPipeline: support for subclass-tuned tokenizer max length, light text cleaning, x-prediction flow matching (x0 -> velocity conversion), and non-unit initial noise scale
• PRXPixelPipeline: thin subclass wiring the above together (vae optional/None, vae_scale_factor=1, 1024px default)
• conversion script support for the pixel checkpoint format
• registration in init files + dummy objects, docs autodoc entry, fast pipeline tests

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Who can review?

@dg845

[HuggingFaceDocBuilderDev]

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[dg845]

Thanks for the PR! Left an initial design review :).

[github-actions]

Hi @DavidBert, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

[dg845]

I think the vae should be removed rather than retained as an optional component for PRXPixelPipeline. My understanding is that if a non-trivial VAE is supplied with either vae_scale_factor > 1 or self.vae.config.latent_channels != self.transformer.config.in_channels, the code will break.

[DavidBert]

Good catch — agreed. vae has been removed entirely from PRXPixelPipeline (the __init__ signature, register_modules, _optional_components, and model_cpu_offload_seq). vae_scale_factor is now a simple property returning 1. The Hub checkpoint's model_index.json also needs the "vae": [null, null] entry removed so from_pretrained continues to work — updating that separately.

[dg845]

Thanks for iterating! Left some follow up comments.

[dg845]

Thanks for your work on this PR!