Assert there are no outstanding side effects before calling cond (#90208)
The current cond implementation is silently incorrect when
there are outstanding side effects, since the locally tracked
side effects are lost when the recursive export call is made.
At least we raise an assert now.
I'm working on a refactor of cond which should be able to sidestep
this problem. Maybe.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: [D41746973](https://our.internmc.facebook.com/intern/diff/D41746973)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90208
Approved by: https://github.com/voznesenskym
