From #6787:

The autoscheduler to use when compiling is currently controlled by a global, which is terrible and also not-threadsafe. (to be fair, there are probably plenty of other places in our compiler that also aren't threadsafe, but no point in adding to them.)

really, the right fix here is to change the auto_schedule GeneratorParam from a bool into a string and (eventually) nuke the 'global' setting entirely