Sapiens2: Remove modeling imports in image processing
Author: guarin
Pull request: yes

# What does this PR do?

* Remove imports from `modeling_sapiens2.py` in `image_processing_sapiens2.py` 
* Remove typings for sapiens2 specific output classes

The purpose of this PR is to remove the unchecked imports from `modeling_sapiens2.py` inside `image_processing_sapiens2.py`. This happened because I typed the post-processing methods with the sapiens2 `ModelOutput` classes which were then automatically added as imports by the modular generator. All other image processors use an `if TYPE_CHECKING` guard for these types of imports (e.g. [here](https://github.com/huggingface/transformers/blob/562021d4ecffcc4d572f9da930ad02d351cb55c8/src/transformers/models/vitpose/image_processing_vitpose.py#L50)). I don't think there is currently any way to instruct the modular generator to guard these imports automatically.

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_46674). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

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

run-slow: sapiens2

run-slow: sapiens2

**CI Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46674)

[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27603309938)

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

models: ["models/sapiens2"]
quantizations: []

## CI Results
[Workflow Run ⚙️](https://github.com/huggingface/transformers/actions/runs/27603309938)

### Commit Info
| Context | Commit | Description |
|---------|--------|-------------|
| RUN | [137a4a8c](https://github.com/huggingface/transformers/commit/137a4a8cd50a8eb1ffc92b873736a53684500213) | workflow commit (merge commit) |
| PR | [6c5d2657](https://github.com/guarin/transformers/commit/6c5d2657f5a468645242a89cb610e849d9218ccc) | branch commit (from PR) |
| main | [7ff490ac](https://github.com/huggingface/transformers/commit/7ff490aca20f597f6c6e42a449c7c8dd28807c6b) | base commit (on `main`) |

✅ No failing test specific to this PR 🎉 👏 !