FR: autoscheduler should strip all manual schedules

[steven-johnson]

Currently, you must structure a Generator with something like if (!auto_schedule) {...} wrapped around your scheduling code, otherwise the autoscheduler's work conflicts with the manual schedule.

If would be much more convenient if the autoscheduler could just strip out all manually-applied scheduling without the need for this structure; that would allow any Generator with appropriate estimates to be used directly.

(This change isn't a big deal for 'simple' Generators, but for complex ones composed of many code pieces, adding the necessary check-for-auto-schedule can be a nontrivial change, as @dsharletg can attest)

[benoitsteiner]

It would be even better if the autoscheduler could start from an existing (potentially partial) schedule, but that might be asking too much. Simply stripping an existing schedule would solve a number of problems for us, so I second this FR ;)