[docs] [testing] distributed training
We figured out how to support distributed training with `pytest`, this is a preliminary doc snippet to help those in need to find the current implementation. I'm sure it will evolve as we  have more tests with varying needs, but for now that's all we have.

@sgugger 
One followup is to update the `test_trainer_distributed` to work with pytest. Then ideally, if we could have one command to run all those tests, that would be awesome (maybe we can use a pytest marker to mark all distributed-specific tests so it's easy to select them all?)<|||||>I will port `test_trainer_distributed` - thank you for flagging that, @sgugger 
Tracking it here: https://github.com/huggingface/transformers/issues/8058