Improvements to gen_sample_etrecord (#1073)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1073

Various improvements to the gen_sample_etrecord script.
- Enable users to generate an etrecord from any of the sample models
- Output files to /tmp instead of the working directory, provide a CLI option for this
- Clean up imports and add type hints

Reviewed By: Jack-Khuu

Differential Revision: D50579104

fbshipit-source-id: 7543f274b0ce9ee96a63fc4f8e8e9bdddebabfd5