FR: there should be a way for autoschedulers to ignore hand-scheduled Funcs

[steven-johnson]

It would be extremely useful to be able to have an autoscheduler apply to only part of a pipeline, with the use case here being a Func that has a carefully hand-optimized schedule that is manually written.

This would save compiler time (no need to run the autoscheduler on as much code) and allow for gradual introduction of hand-optimization to an autoscheduled pipeline, rather than this all-or-nothing approach we have now.

Conceptually this could be handled in the same way as a define_extern() Func (#4101), though exactly how such a "don't touch me" Func would be specified syntactiaclly would be TBD.