Rewrite fallthrough to more closely match how C++ works (#96304)
Fallthrough is modeled as a mask which we use to remove keys from the
compute dispatch key set for eligibility.
It's possible this addresses https://github.com/pytorch/pytorch/issues/89037
in a better way than https://github.com/pytorch/pytorch/pull/95891 but I
cannot easily tell as the original repro no longer works and the new PR
does not have a test.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96304
Approved by: https://github.com/zou3519, https://github.com/albanD, https://github.com/zhxchen17
