Update export api in codebase to long term version (#999)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/999

Current bundled program adopts legacy export apis such as `exir.capture_multiple` to export ET model. This diff updates our codebase to use long term API.

Reviewed By: JacobSzwejbka

Differential Revision: D50281486

fbshipit-source-id: 96b60ef4d519b9e5b19cd46a41459f16318c80df