All autoschedulers should be moved to a single location

[steven-johnson]

Currently we have the 'classic' autoscheduler baked into Halide, and the 'ML' autoscheduler living in apps/autoscheduler; soon we will also have the 'gradient' autoscheduler living in apps/gradient_autoscheduler.

We really need to refactor all of these so that:

• all are plugins ('classic' shouldn't be privileged anymore)
• all live somewhere other than apps/ (e.g. a new toplevel plugins/ folder or some such)
• all are built into distrib/ folder for binary distributions
• validation testing for all of them is available as part of the normal test suite (rather than via apps/)

[steven-johnson]

Ack, dupe of #4349 #4053

[BachiLi]

Do we want to open this and close #4341? This seems much more informative.

[steven-johnson]

#4341 may or may not be worthy of a separate issue. Definitely leave this one closed as it's a dupe of #4349 #4053

[BachiLi]

But this is exactly #4349 ..?

[steven-johnson]

Whoops, copy-paste error! I meant #4053