Only import torch.distributed if it is available

[GaetanLepage]

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 huggingface/accelerate#2121 to load torch.distributed conditionally on its availability.
I was able to test the patch in nixpkgs successfully.

Fixes huggingface#35129

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