all_gather_into_tensor is deprecated, _gpu_gather warns on every gather

### System Info

accelerate 1.15.0, torch 2.13.0+cu129, transformers 5.18.0.dev0, 8 x H100.

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Reproduction

`_gpu_gather` uses an API torch deprecated, so every `Accelerator.gather` logs a FutureWarning once per rank:

```
torch/distributed/c10d_logger.py:83: FutureWarning: `torch.distributed.all_gather_into_tensor` is deprecated.
Please use `torch.distributed.all_gather_single` instead.
```

```python
# utils/operations.py:324
gather_op = torch.distributed.all_gather_into_tensor
```

It is reached from any Trainer run, `Trainer._get_num_items_in_batch` -> `Accelerator.gather` -> `_gpu_gather` -> `_gpu_gather_one`.

### Expected behavior

No warning. `torch.distributed.all_gather_single` exists in torch 2.13, so this can be a rename behind a version check for older torch.

Start in utils/operations.py around line 324 and trace _gpu_gather through _gpu_gather_one, then check the reported torch 2.13 API and older-version compatibility. The work is done when Accelerator.gather no longer emits the deprecation warning while remaining compatible with supported torch versions.

Written by the indexing model from the issue text.