[discrete diffusion] Add DiffusionGemma pipeline and schedulers

[kashif]

Adds a DiffusionGemma block-diffusion pipeline, alongside the schedulers already on this branch (discrete DDIM, entropy bound, and a uniform mode for block refinement).

DiffusionGemma is an encoder-decoder block-diffusion model: the encoder reads the prompt into a KV cache and the decoder denoises a fixed-size canvas by cross-attending to it. The pipeline runs the outer canvas loop and the inner denoising loop, sampling candidates each step, committing the most confident ones via BlockRefinementScheduler in uniform corruption mode, and renoising the rest. Structure mirrors the LLaDA2 and dflash (#13699) pipelines.

The model itself lives in transformers as DiffusionGemmaForBlockDiffusion (released in 5.12.0).

Tested:

• pipeline unit tests pass (plumbing, callbacks, output types)
• the pipeline drives the real tiny checkpoint end to end without error

Quality on the full google/diffusiongemma-26B-A4B-it checkpoint still needs a GPU run.

[HuggingFaceDocBuilderDev]

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[zucchini-nlp]

Looking great! A couple questions from quick skimming

[yiyixuxu]

thanks for the PR! i left a few comments

I reviewed this through the lens of diffuser convention/style. If some of these choices are intentional to keep things familiar for Transformers users, let me know, and we can figure out the right balance together

[yiyixuxu]

I think between prompt and messages, we only need accept prompt since it's a really cheap into messages

it's just this, no?

messages = [{"role": "user", "content": prompt}]

[kashif]

Makes sense. The one wrinkle is image prompts, which we pass through messages today, so I'll fold the prompt/messages simplification into the image input rework so single-image and text both stay clean. Coming in a follow-up.

[kashif]

Made prompt the primary input and dropped the tokenized intermediates. Kept messages for raw multi-turn/multimodal conversations (per the thread below with zucchini), and added a raw image arg for the simple prompt+image case, so it is all raw inputs now.