What does this PR do?

Simple tweak to skip initialization of the torch process group if one is already initialized.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• [ ] Did you write any new necessary tests?

Who can review?

@OlivierDehaene

In what setting is this called twice? In my oppinion this should crash.

If the model is ran in a process which already has the process group initialized by another library (eg. deepspeed, Ray), I don't think it should crash.

If you want to throw an exception by default, then perhaps this behavior could be put behind an env var, or made overridable in the model classes themselves?

From an architectural perspective, it doesn't seem right for the process group to be initialized in the model itself. The more common pattern is to initialize it separately (see eg. torchrun)

@OlivierDehaene can you let me know if we can proceed here or if you think this PR should be rejected? It would be helpful for my use case if we could merge this. Thanks.