refactor the models in test_model.py (#3459)

Summary:
Pull Request resolved: #3459

# context
* refactor the models in test_utils.test_models
* add docstring
* absorb optional arguments into `**kwargs`

Reviewed By: spmex

Differential Revision: D84615254

fbshipit-source-id: 9f4b793534c24cce969717da4511d07b8abf8c11

Author: TroyGarden