Better management of __init__ files

I can't do this:

```python
from tf2_yolov4 import YOLOv4
```

When I do this, it fails:

```python
import tf2_yolov4

yolo_model = tf2_yolov4.model.YOLOv4
```

I think it's related to our __init__ files being empty. We should add the `__all__` export in those init