Label Smoothing for Question Answering task
# 🚀 Feature request

Currently label smoothing is only applied when "label" is present in the input in compute_loss function and not in case of question answering default in Trainer class.

## Your contribution

I would like to work on this issue and submit a PR by modifying compute_loss to use label_names to apply label smoothing for question answering. 
cc @sgugger <|||||>I'm not entirely sure how straightforward this could be, plus it seems like a very narrow use case. I think this should be implemented independently by the user with a custom `compute_loss`  function.<|||||>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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