fix: support tensor labels in DataCollatorWithFlattening
Author: hqkqn32
Pull request: yes

## Description

Fixes #42502

This PR adds support for `torch.Tensor` labels in `DataCollatorWithFlattening`. Previously, the collator only worked with list labels and raised a `TypeError` when tensor labels were provided.

## Problem

When users passed `torch.Tensor` objects as labels to `DataCollatorWithFlattening`, it failed with:
```python
TypeError: can only concatenate list (not "Tensor") to list
```

This happened because the collator used list concatenation (`+=`) without checking if inputs were tensors.

## Solution

- Added type checking using `hasattr(obj, "tolist")` 
- Convert tensors to lists before concatenation
- Applied to both `input_ids` and `labels`
- Maintains backward compatibility with list inputs

## Changes Made

- Modified `src/transformers/data/data_collator.py`:
  - Added tensor-to-list conversion in `DataCollatorWithFlattening.__call__()`
- Added tests in `tests/trainer/test_data_collator.py`:
  - `test_flattening_with_tensor_labels`: Verifies tensor labels work
  - `test_flattening_with_list_labels`: Regression test for list labels

## Testing

### Before this PR ❌
```python
features = [
    {"input_ids": torch.tensor([1, 2, 3, 4]), "labels": torch.tensor([10, 11, 12, 13])},
    {"input_ids": torch.tensor([5, 6, 7]), "labels": torch.tensor([14, 15, 16])},
]
collator = DataCollatorWithFlattening(return_tensors="pt")
batch = collator(features)  # TypeError!
```

### After this PR ✅
```python
features = [
    {"input_ids": torch.tensor([1, 2, 3, 4]), "labels": torch.tensor([10, 11, 12, 13])},
    {"input_ids": torch.tensor([5, 6, 7]), "labels": torch.tensor([14, 15, 16])},
]
collator = DataCollatorWithFlattening(return_tensors="pt")
batch = collator(features)  # Works perfectly!
```

## Checklist

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] Tests added for the fix
- [x] All new and existing tests passed
- [x] Code follows the project's style guidelines

Comments:
(Give the CI a little bit and try again, I think it's just an intermittent issue)

@Rocketknight1  Thank you for the clarification! I'll trigger the CI again with an empty commit.


I see some test failures related to Mistral tokenizer, but these don't seem related to my DataCollator changes. The failing tests are about tokenizer templates, not data collation. 

Should I be concerned about these, or are they existing issues in the base branch? @Rocketknight1 

I suspect these are in the base branch, I'll see if I can do something about them!

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=42620&sha=96f8e2

cc @ydshieh this seems like a CI issue - those tests are passing locally for me. It looks like the CI is loading the wrong class, or possibly has an out-of-date cached version of the `hf-internal-testing/namespace-mistralai-repo_name-Mistral-Small-3.1-24B-Instruct-2503` repo.

The repo. has no changes since its creation, so unlikely a cache issue. I don't know if the repo. should be updated after the tokenizer refactorization however. I can merge this PR.