[TOPI][x86] Introduce schedule_injective_from_existing and unify external schedules for all targets (#3983)

* Fix extern schedule for x86

* Register x86::schedule_extern

* Fix

* Fix

* Replace extern.py with extern.h

* Introduce new generic function schedule_injective_from_existing

* Fix

* Fix

* Add back to C++

* Fix style

* Injective schedule calls local schedule_injective_from_existing

* Fix

* Remove target arg from schedule_injective_from_existing

* Fix docs

* Try to fix unit test

* Fix test

* Fix other tests

* Fix bug