flow generator tests

[clewis7]

https://github.com/jbohnslav/deepethogram/blob/master/tests/test_flow_generator.py

can't really test training...but can test:

• [ ] passing in default checkpoint vs non-default checkpoint
• [ ] loading model from default for all modes
• [ ] checking that lightning module setup worked
• [ ] checking dataloaders/dataset stuff

[clewis7]

one of the inputs to flow_generator training is gpu_id...need to think more about how to write tests that would go in CI when CI does not have gpu

https://github.com/github/roadmap/issues/505