[DiffusionGemma] return_dict_in_generate=False for image-text-to-text…
Author: KoichiYasuoka
Pull request: yes

… pipeline

# What does this PR do?

Fixes #46701 

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## Who can review?

@zucchini-nlp
@Rocketknight1



Comments:
Here I have two ways to resolve the tests_torch errors:
 - `return_dict_in_generate=True` for default behavior
 -  edit test sets

Which one is better for @kashif and @zucchini-nlp ?

Thank you @zucchini-nlp I changed return_dict_in_generate=True as default, and I've just written a small test for return_dict_in_generate=False.

**CI Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46724)

@Rocketknight1 I'm waiting for your review on [image_text_to_text.py](https://github.com/huggingface/transformers/pull/46724/files/2fc515aeefcdd83883f7ca059c4d74c3f5d38b88#diff-6902c688132722500bd569c3fd58d9de175551d2622d0639f9ec5aaf6c2b839c).

Lets remove the `return_dict` from hardcoded defaults and we can merge

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

run-slow: diffusion_gemma

run-slow: diffusion_gemma

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/28015398772)

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_46724). 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/28015398772)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [fe7f0f8b](https://github.com/huggingface/transformers/commit/fe7f0f8b55bd90a19b8b45a0fa224c5709fd782c) | workflow commit (merge commit) |
| PR | [67e7bbe4](https://github.com/KoichiYasuoka/transformers/commit/67e7bbe41951e2d5578b1986a9ae2c2ce052620a) | branch commit (from PR) |
| main | [4d47b061](https://github.com/huggingface/transformers/commit/4d47b06102805e02662d5d9865994bed8aa6d7fb) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !