Make DispatchKeyGuards accept DispatchKey::Undefined (#32729)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32729
When working on the vmap prototype I noticed that this was helpful
as it lets me easily initialize a no-op guard, if I need to do it
at constructor time (which I usually do, because the guards don't
have move constructors).
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D19628092
Pulled By: ezyang
fbshipit-source-id: d6259a3f70d287cdac2e4a5f3984e2880f19bdc2
