[doc] Add missing "Returns" sections in docstrings

src/mednet/config/data/tbx11k/datamodule.py

src/mednet/config/data/tbx11k/make_splits_from_database.py

src/mednet/data/augmentations.py

src/mednet/data/datamodule.py

src/mednet/data/split.py

src/mednet/data/typing.py

src/mednet/engine/device.py

src/mednet/engine/saliency/generator.py

src/mednet/utils/rc.py

src/mednet/utils/resources.py

tests/conftest.py