Potential fix for #30819. Check for best metric only on gpu 0
Author: godspeed5
Pull request: yes

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Fixes #30819\. Allows running question-answering with DDP using torchrun by doing metric_for_best_model checks on only the main process. I have tested it on question-answering but maybe this needs unit tests in the CI too?


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Comments:
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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Hey, just bumping this up. Is this fix reasonable?

Hey, thanks for the review @muellerzr ! 

About using broadcast, some (actually maybe most?) examples compute the metrics on all processes. question-answering does it only on one process though ([metric computation in run_qa](https://github.com/huggingface/transformers/blob/16ed0640be71cd38208eed87bdcf39be29a83b5d/examples/pytorch/question-answering/run_qa.py#L633), [HF evaluate running metric computation only on process 0](https://github.com/huggingface/evaluate/blob/5be95dfebca7040b911950f5ece562c2d8da142a/src/evaluate/module.py#L462)). Summarization in contrast does it on all processes but [compute_metrics is written in the run script](https://github.com/huggingface/transformers/blob/b01f9c484cd24596b274589222746446372ce043/examples/pytorch/summarization/run_summarization.py#L645) and doesn't use HF evaluate. 

I was thinking about broadcasting but I think it would only work if using Evaluate and not for Summarization type examples where metric computation happens on all processes.

 If I understand correctly, the metric computation should have the same value on all processes for a Summarization-style example by the time the program control reaches the points where I inserted the checks. 

Please let me know if this makes sense or if I'm wrong anywhere.

Also, could you elaborate on backend agnostic please? Was a process-id check like that used in HF Evaluate what you meant as opposed to torch cuda based checks? 

Bumping this up in case this fell through the cracks