Refactor pull workflow to avoid code duplication and add quantization export test (#84)

Summary:
This PR addresses the second part of T160762924 and refactors the pull workflows in the following parts:

* Make buck2 and cmake part of the build matrix, so we don't need to have 2 separate jobs for them
* Run `test-custom-ops-[linux,macos]` in their own jobs.  Atm, they are run multiple times for each example models
* Add quantization export for supporting [models](https://github.com/pytorch/executorch/blob/main/examples/quantization/example.py#L99)

Pull Request resolved: https://github.com/pytorch/executorch/pull/84

Test Plan: https://github.com/pytorch/executorch/actions/runs/5931450236

Reviewed By: larryliu0820

Differential Revision: D48534520

Pulled By: huydhn

fbshipit-source-id: 33dbb4033e23cae5f5677a8dd518f09295c7ad9f