Make executor_runner use MethodMeta (#318)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/318

Using MethodMeta lets us avoid the off-by-one hack used by the (deprecated) Program versions of the buffer iterators.

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: D48762562

fbshipit-source-id: a06121c0841e97180c535162b1cd8854357aa2fb