Don't load empty directory in unit tests (#823)
Summary:
In development, sometimes there will be empty directories left in the `models` directory (usually caused by branch switching). Only load the model directories that have a `__init__.py` file, which means it is not empty.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/823
Reviewed By: erichan1
Differential Revision: D35127882
Pulled By: xuzhao9
fbshipit-source-id: 88115ce702169c4d14d0a4ce25504189a34cff8b
