Fix batched DiffusionGemma adaptive stopping

[kashif]

DiffusionGemma adaptive stopping currently waits for the whole batch to converge, so rows that finish early keep changing while slower rows continue. It also measures confidence from the raw model logits instead of the scheduler-shaped logits used for sampling.

This freezes each row once it is stable and confident, and uses the scheduler logits for the stopping decision. A regression test covers both cases.

Checks:

• make quality
• make fix-copies
• DiffusionGemma pipeline tests (11 passed)
• compiled decoder with static cache smoke test

[HuggingFaceDocBuilderDev]

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[dg845]

Thanks for the PR! Are the changes (committing finished rows immediately, using scheduler-shaped logits instead of raw denoising model logits) intended to match the DiffusionGemma reference behavior? It's not obvious to me that they are bugfixes rather than modeling changes.

[kashif]

thanks @dg845 fixing!