Allow each model to choose its own runner (#277)

Summary:
It's a good idea to allow each model to choose its own runner.  This also help unblock https://github.com/pytorch/executorch/pull/212 until we know the root cause of why it runs OOM.

Pull Request resolved: https://github.com/pytorch/executorch/pull/277

Reviewed By: kirklandsign

Differential Revision: D49176509

Pulled By: huydhn

fbshipit-source-id: b5e74943fb62839e3b9a7ff132842c8614d0f84b