[Qwen3MoE] Fix wrong return type annotation in Qwen3MoeSparseMoeBlock.forward

[matdou]

Fixes #45208

What does this PR do?

This PR corrects an incorrect return type in Qwen3MoeSparseMoeBlock.forward.

The method was annotated as returning tuple[torch.Tensor, torch.Tensor], while the implementation returns a torch.Tensor:

return final_hidden_states.reshape(batch_size, sequence_length, hidden_dim)

In particular, downstream usage (e.g. in Qwen3MoeDecoderLayer) expects a tensor:

hidden_states = self.mlp(hidden_states)
hidden_states = residual + hidden_states

This PR updates the annotation to:

-> torch.Tensor

in both:

• modeling_qwen3_moe.py
• modular_qwen3_moe.py

No functional changes are introduced, this is a typing correction only.

---

Checklist

• I confirm that this is not a pure code agent PR.
• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker @Cyrilvallez

[github-actions]

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

run-slow: qwen3_moe, qwen3_omni_moe, qwen3_vl_moe

[Rocketknight1]

No drive-by code agent PRs on other people's issues please!