Add backend to InitProcessGroupKwargs

[alex-hh]

To allow users the choice between e.g. gloo and nccl Is there any reason not to support this?

[sgugger]

I don't see why not, if you want to make a PR adding support :-) Initially I didn't include it because users can always do the init on their own and in this case Accelerate won't re-do it. But since there are all the other kwargs now...

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