[trainer] add distributed_env to TrainingArguments
As discussed in https://github.com/huggingface/transformers/pull/8823 it's not simple to check whether the downstream code is running under distributed mode or not (currently requires checking `self.args.local_rank != -1` which is far from obvious.

So we were discussing about adding a flag like `distributed_env` so that the downstream code could do a much simpler intuitive check.

I'm not sure whether we need just True/False for ddp or whether we also need to have another flag if we are under DP as well?

@sgugger 