Refactor ExportPassBase. (#1532)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/116778

Pull Request resolved: https://github.com/pytorch/executorch/pull/1532

as title. This diff decouple the pass base library from torch export and exir, so that different layers can evolve in their own fashion, and we have more head room to divide and conquer in the future.

(Note: this ignores all push blocking failures!)

Reviewed By: angelayi

Differential Revision: D52514517

fbshipit-source-id: 930d3bed8b2655632ffe07a44f3033f3c40e2ce0