Add AoT apis (#345)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/345

Added long term aot api for executorch.

Consists of 3 stages based on the dialect the program is in,

ATen
Edge
Executorch

Unified api for multiple entry point and single entry point

Reviewed By: mergennachin, kimishpatel

Differential Revision: D49163694

fbshipit-source-id: 9ae7374e9c53db34f815dc61d84653f1d7e7a049