remove sequence parallel in llama4
Author: 3outeille
Pull request: yes

# What does this PR do?

This PR fix the issue #39835 where a `RuntimeError` was triggered due to mixing tensor and Dtensor. This is due to `_prepare_output_fn` being force to returns a tensor instead of Dtensor.

```
    @staticmethod
    def _prepare_output_fn(output_layouts, use_local_output, mod, outputs, device_mesh):
        outputs = outputs.redistribute(
            placements=(Replicate(),), async_op=True
        )  # maybe we have to replicate ? because next layer is not sharded
        return outputs.to_local()
 
```

An easy fix could be to just to return a Dtensor everytime. However, after some discussion, it is better to remove the `SequenceParallel` use for now as it slows down the overall generation.

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Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_40084). 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: llama4