Add more sample models to OSS CI (#76)

Summary:
This updates the CI to build and test all models under `examples/models`.

* A simple script `.ci/scripts/gather_test_models.py` gathers all the models there and makes them available to the CI.  All models are then run in parallel.
* Install vision and audio as they are needed by `mobilenet` and others

We have 8 current models x 2 os (linux and mac) x 2 builds system (buck2 and cmake) = 32 combinations.  I will check with the team on the status of quantized models testing and add them in a later PR.

Pull Request resolved: https://github.com/pytorch/executorch/pull/76

Reviewed By: larryliu0820, guangy10

Differential Revision: D48485199

Pulled By: huydhn

fbshipit-source-id: d78ee063b89e5be0d0d6ea5757e3c3e72d83feb1