add modular blockset for diffusion gemma

[akshan-main]

What does this PR do?

Adds a modular blockset for DiffusionGemma: DiffusionGemmaBlocks + DiffusionGemmaModularPipeline, with a text encoder step, a generation setup step (canvas layout, KV cache, EOS state), a scheduler setup step that splits the budget into predictor and corrector steps, a canvas loop that prefills, denoises and appends each block, and a decode step that trims at EOS and detokenizes. It is the first modular pipeline with text output. The checkpoint keeps its weights at the repository root, so the docs build the components by hand as in the standard usage.

Outputs match DiffusionGemmaPipeline token for token with shared components and seed, on the tiny components and on google/diffusiongemma-26B-A4B-it in bf16 (one and two canvases, temperature sampling, a batch of two prompts of different lengths, and the static cache path). The tests include the batched adaptive-stopping cases from #14386. Test components live in akshan-main/tiny-diffusion-gemma-modular-pipe.

The first commit is the loader fix from #14661 (a transformers model folder that shadows a pipeline folder resolved to the wrong library, which broke save and reload here) and drops out once that lands. #14661 also makes the scheduler accept CPU generators on CUDA, which the GPU runs of the modular tests need.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Discussed on Slack with @sayakpaul @yiyixuxu
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@yiyixuxu @sayakpaul

[github-actions]

Hi @akshan-main, 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.

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