Only import torch.distributed if it is available
Author: GaetanLepage
Pull request: yes

# What does this PR do?

`torch.distributed` is sometimes unavailable (on some platforms such as darwin).
It leads to `transformers.models.auto.modeling_auto` to fail to import with:
```
ModuleNotFoundError: No module named 'torch._C._distributed_c10d'; 'torch._C' is not a package
```

I propose to implement a similar logic as in https://github.com/huggingface/accelerate/pull/2121 to load `torch.distributed` conditionally on its availability.
I was able to test the patch in [nixpkgs](https://github.com/NixOS/nixpkgs/) successfully.

Fixes #35129


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Comments:
cc @kwen2501 @ArthurZucker - I believe this is caused by the imports added in https://github.com/huggingface/transformers/pull/34184

> cc @kwen2501 @ArthurZucker - I believe this is caused by the imports added in #34184

Yes, indeed. We vendored a simple patch in nixpkgs: https://github.com/NixOS/nixpkgs/pull/362768/files.
It simply skip the import on darwin and this is good enough for us now.
All the downstream packages depending on `transformers` seem to work fine.