Add "Loader=..." to yaml.load callsites (#588)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/588

Calling `yaml.load` without `Loader=` is deprecated and will throw in `pyyaml` 6.x. [The behavior in 5.x is to use `Loader=yaml.FullLoader`](https://github.com/yaml/pyyaml/blob/5.4.1.1/lib/yaml/__init__.py#L110), so we retain that semantic.

Differential Revision: D49670999

fbshipit-source-id: b22b574a51f84f3bebf88f8d1246de4582101362