fix: DataCollatorWithFlattening incompatible with Tensor input ids
Author: sudhanshu746
Pull request: yes

# What does this PR do?
This PR provides a bug Fix for List Concatenation Error in __call__ Method in DataCollatorWithFlattening
Fixes # (https://github.com/huggingface/transformers/issues/33946)

## Problem Description
   The error traceback:
`TypeError: can only concatenate list (not "Tensor") to list `

## occurs at the following line of code:
  `line: 1649   ret["labels"] += [separator_id] + features[idx]["input_ids"][1:]`

## How to reproduce the error ?

```
from transformers import GPT2Tokenizer, DataCollatorWithFlattening
tokenizer = GPT2Tokenizer.from_pretrained("openai-community/gpt2")
example = tokenizer("A test sentence", return_tensors="pt")
collator = DataCollatorWithFlattening()
batch = collator([example]*2, return_tensors="pt")
print({k: v.tolist() for k, v in batch.items()})
```


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? 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](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@ArthurZucker


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Comments:
Hi @ArthurZucker,  I'm happy to start with my Good first PR for active contribution to the Transformer library by fixing an issue. It's a way to give back to the beloved project I use. Looking forward to collaborating more and learning together!

Can you please help me to fix [check_repository_consistency](https://app.circleci.com/pipelines/github/huggingface/transformers/108140/workflows/bd7a3637-4cca-46c3-95d6-889fa7c7769c/jobs/1436798?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-checks-link&utm_content=summary) job ?


@ArthurZucker  kindly review the PR. The repo has been synced and resolve import issue.

Hi @sudhanshu746, I'm sorry, there are still some issues with this PR, and all the CI tests are red!

The main cause is that you're importing torch at the top of the module, instead of in the functions that use it. The reason that fails is that this code needs to work on TensorFlow/Flax systems as well, which may not have Torch installed. Therefore, we only import torch in the torch functions. If you revert those changes, I think the CI should be green - you can take a look at the CI errors by clicking the "Details" button by each one to see what errors it's seeing.

@Rocketknight1, Hi Mat, I'm not sure why still merge is still failing even after the fix.