We can probably remove all Unsupported subclasses in exc.py and replace the `raise <Unsupported subclass>` callsites with `unimplemented`.

For example, `torch._dynamo.exc.UserError` was used for ExportDB before we converted all Dynamo graph breaks to the new format.

cc @chauhang @penguinwu @voznesenskym @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @kadeng @amjames @Lucaskabela @jataylo