Add support for `ModernBertForMultipleChoice`
Author: netique
Pull request: yes

# What does this PR do?

This PR implements `ModernBertForMultipleChoice` class that was missing.


## Who can review?

@ArthurZucker

Comments:
Hi @netique, I was looking for this and noticed you had already done it! Seems like it's close but lmk if you need any help, excited to have it in :) 

Hi @netique just following up to see if I can help! 

If I don't hear from you, I'll assume you're busy and submit a similar PR. It seems like you mostly just need to make sure you pass the tests/formatting by propagating the same code to the `modeling_*` file also.  If helpful feel free to add me as a contributor to your fork and I can push that.

@orionw Hi, thanks for the follow-up. I did everything as https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md told me but I cannot get `modeling*` to reflect the changes from `modular*`. I keep getting.

```
ValueError: Some diff and their modeling code did not match. Models in diff:
 - modernbert
```

Ah yeah I've noticed that too. There's probably a real way to do it but what I do since it's quick is just literally copy it in so they match. It doesn't seem to automatically reconcile it for me either.

Then re-run the check to see if there's still a diff (it should output a diff you can see). If not, you're good to go!

Looks like you resolved that @netique!

The last thing looks like the tests (it requires the new class has tests). If you add it to the imports in you can add a test like:
```
    def create_and_check_for_multiple_choice(
        self, config, input_ids, input_mask, sequence_labels, token_labels, choice_labels
    ):
        config.num_labels = self.num_labels
        model = ModernBertForMultipleChoice(config=config)
        model.to(torch_device)
        model.eval()
        result = model(input_ids, attention_mask=input_mask, labels=choice_labels)
        self.parent.assertEqual(result.logits.shape, (self.batch_size, self.seq_length, self.num_labels))
```

And then you'll also have to add it the lines in [this test](https://github.com/huggingface/transformers/blob/main/tests/models/modernbert/test_modeling_modernbert.py#L228) wherever the `TokenClassification` class is.

Again, feel free to add me as a collaborator on the repo if you don't have time to finish, happy to help!

Thanks @orionw! I had to rewrite it to be compatible with the output, as the multiple-choice tasks are more complicated. Also, the test must be "registered," and I had to resolve the `.md` docs. Now it looks like we are good to go!

Amazing, really excited for this to be merged in! Thanks @netique!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39232). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@ArthurZucker / @Rocketknight1 I think this was approved and passes all tests - I think you just need to merge :) 

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

run-slow: auto, modernbert

@ArthurZucker a ping to merge since you already approved / everything passes tests.

Thanks for your patience everyone! 🤗