[easy] fix pytest error related to test name conflict

[hudeven]

to reproduce the error, run pytest at "recipes/torchrecipes" and will get error like:

import file mismatch:
imported module 'test_train_app' has this file attribute:
/Users/stevenliu/workspace/torchrecipes/recipes/torchrecipes/audio/source_separation/tests/test_train_app.py
which is not the same as the test file we want to collect:
/Users/stevenliu/workspace/torchrecipes/recipes/torchrecipes/vision/image_classification/tests/test_train_app.py

Adding __init__.py fixed the issue

[facebook-github-bot]

@hudeven has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@hudeven has updated the pull request. You must reimport the pull request before landing.