Declare the LAPACK and MAGMA dispatchers instead of defining them with a default error (#28133)
Summary:
This clears a lot of dead code that isn't reachable due to `AT_DISPATCH`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28133
Test Plan: - All existing tests should pass to ensure that the change is valid.
Differential Revision: D17978803
Pulled By: ezyang
fbshipit-source-id: 8fdaa74f9addb1d7987c5d625557b8a463a25500
