structural api and schema update (#1009)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1009

Update the AOT API for bundled program to make it more user friendly.

We created two new classes for holding testcases in a more structural way:

MethodTestCase: contains inputs and expected outputs for a single test for method.

MethodTestSuite: Collection of all test cases for a program method.

Detailed design can be found here: https://docs.google.com/document/d/170WJ81dPuNQsvsjqH_wQjafbu2kxtEG1sgX-z6OqCEI/edit#heading=h.69fgezruwl6n

Reviewed By: tarun292

Differential Revision: D50422563

fbshipit-source-id: 6bef1d29f4f2d5d56eacbb6744283066d40724e8