Flatten exceptions in dynamo (#100779)
Fixes https://github.com/pytorch/pytorch/issues/93571
[before and after](https://gist.github.com/mlazos/256b0e8f0f98495752a22b960e9f4fcb)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100779
Approved by: https://github.com/ezyang
