DiffusionGemma: mask layout and CI
Author: zucchini-nlp
Pull request: yes

# What does this PR do?


PR also changes the mask layout to be more intuitive. Mask is simply bidirectional, but we needed a few things to make it work with static cache and prefix-tuning format. Now the static cache mask will be a right-padded bidirectional mask, instead of padding in the middle

For advanced users it might be breaking: please prepare the mask with `model.create_diffusion_decoder_attention_mask` to get it correctly and pass the whole config to it (not the text config)

 Slow CI passes now, the adjustment of expectation was done based on `main`, so we can trust the values.


Comments:
run-slow: diffusion_gemma

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27536806680)

This comment contains `run-slow`, running the specified jobs: 

models: ["models/diffusion_gemma"]
quantizations: []

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_46654). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27536806680)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [abc0b82d](https://github.com/huggingface/transformers/commit/abc0b82d4f9f47387b64d7abf8ec97b5dba02ee7) | workflow commit (merge commit) |
| PR | [bd4cf3e0](https://github.com/zucchini-nlp/transformers/commit/bd4cf3e031b1ff98474a11ebed7bfc8c1046ce00) | branch commit (from PR) |
| main | [d242bb79](https://github.com/huggingface/transformers/commit/d242bb790bcbbe6c9a20e46cf9d70648739a90bf) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !


run-slow: diffusion_gemma

run-slow: diffusion_gemma, gemma4

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27625917086)

This comment contains `run-slow`, running the specified jobs: 

models: ["models/diffusion_gemma", "models/gemma4"]
quantizations: []

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27625917086)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [e8a62280](https://github.com/huggingface/transformers/commit/e8a62280391a58c2ddc4e36c1b934e84085d471f) | workflow commit (merge commit) |
| PR | [6be952dc](https://github.com/zucchini-nlp/transformers/commit/6be952dca2431f89658c35fa4e55c5b8a7037ed4) | branch commit (from PR) |
| main | [0a4d1c8b](https://github.com/huggingface/transformers/commit/0a4d1c8bac38208fb5b424a510b8a9cef149d1ee) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !


cc @gante when you're back :)

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=46654&sha=26b83a

**CI Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46654)

run-slow: diffusion_gemma, gemma4

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27827814062)

This comment contains `run-slow`, running the specified jobs: 

models: ["models/diffusion_gemma", "models/gemma4"]
quantizations: []

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27827814062)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [c8909d43](https://github.com/huggingface/transformers/commit/c8909d43d09afdb8c61c002f750f82bbe9ef07e9) | workflow commit (merge commit) |
| PR | [c3f2bdef](https://github.com/zucchini-nlp/transformers/commit/c3f2bdef6f1c028b76c4498d32349dd6568cea18) | branch commit (from PR) |
| main | [c2c13860](https://github.com/huggingface/transformers/commit/c2c13860884e72c5c28e42e38676a326f01642b7) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !


Also rerun slow ci please, it was broken https://huggingface.slack.com/archives/C01NE71C4F7/p1781877684393659

run-slow: diffusion_gemma, gemma4

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: diffusion_gemma, gemma4

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27937497238)

This comment contains `run-slow`, running the specified jobs: 

models: ["models/diffusion_gemma", "models/gemma4"]
quantizations: []

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27937497238)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [8d8e13fd](https://github.com/huggingface/transformers/commit/8d8e13fd9e93c9194f3f8f500f44d18971f4ff79) | workflow commit (merge commit) |
| PR | [bd742ef1](https://github.com/zucchini-nlp/transformers/commit/bd742ef146ce95731a555bde744dc706dec371c2) | branch commit (from PR) |
| main | [7bd2725e](https://github.com/huggingface/transformers/commit/7bd2725e97c786a85180df0dea5441b290a9b1d6) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !


CI is passing, will merge after lunch if no serious comments