Make bundled_executor_runner use MethodMeta interface and adjust non-const indices (#319)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/319

The new MethodMeta interface avoids the need to adjust non-const buffer indices by 1.

While I'm here, stop using runtime_allocator to create the outputs list: it's not legal to touch the runtime_allocator after the Method is created.

Reviewed By: JacobSzwejbka

Differential Revision: D49208343

fbshipit-source-id: 86850941ae1e1d3896e0704130785ccd0a06ed37