Distributed gather crashes on PyTorch < 2.6

### Describe the bug

AttributeError: module 'torch' has no attribute 'accelerator' when running distributed gather on PyTorch versions < 2.6.

This error happens because `gather_size_by_comm` in `src/diffusers/models/_modeling_parallel.py` uses `torch.accelerator.current_accelerator()`, which only exists in PyTorch 2.6+. Diffusers officially supports PyTorch 2.1+, so this causes a crash on versions 2.1–2.5 with `AttributeError: module 'torch' has no attribute 'accelerator'`.

### Reproduction

Since this is a utility function; it can be triggered directly with a minimal distributed setup:

```py
import torch.distributed as dist
from diffusers.models._modeling_parallel import gather_size_by_comm

dist.init_process_group(
backend="gloo",
init_method="file:///tmp/pg",
rank=0,
world_size=1,
)

gather_size_by_comm(1, dist.group.WORLD)

```

### Logs

```shell
[rank0]: Traceback (most recent call last):
[rank0]: File "/home/aja/diffusers/test.py", line 11, in
[rank0]: gather_size_by_comm(1, dist.group.WORLD)
[rank0]: File "/home/aja/diffusers/src/diffusers/models/_modeling_parallel.py", line 293, in gather_size_by_comm
[rank0]: gather_device = "cpu" if "cpu" in comm_backends else torch.accelerator.current_accelerator()
[rank0]: ^^^^^^^^^^^^^^^^^
[rank0]: File "/home/aja/diffusers/.venv/lib/python3.11/site-packages/torch/__init__.py", line 2216, in __getattr__
[rank0]: raise AttributeError(f"module '{__name__}' has no attribute '{name}'")
[rank0]: AttributeError: module 'torch' has no attribute 'accelerator'
```

### System Info

diffuser : `0.37.0.dev0`
torch : `2.4.0`
python : `3.11`
system: Linux

### Who can help?

@sayakpaul @DN6

Start in src/diffusers/models/_modeling_parallel.py at gather_size_by_comm and run the minimal distributed reproduction from the issue with PyTorch 2.4. Trace how the communication backends determine gather_device. Done means distributed gather no longer raises AttributeError on supported PyTorch 2.1–2.5 while retaining behavior on newer versions.

Written by the indexing model from the issue text.