Reviewed DataModule design+docs+types

In this MR, I try to review the use of the data module and go further in simplifications for the whole code:

• Slightly streamlined the datamodule approach
• Added documentation
• Added type annotations
• Added some TODOs for further discussion

Closes #7 (closed).

TODO:

• (@dcarron) Implement/document/type hint normaliser (ensure it is used correctly through all the models; integrate on the train script) and that train CLI works!
• (@andre.anjos) Deal with import reweight_BCEWithLogitsLoss

src/ptbench/data/padchest/__init__.py

@@ -264,7 +264,7 @@ json_dataset = JSONDataset(
 def _maker(protocol, resize_size=512, cc_size=512, RGB=True):
     import torchvision.transforms as transforms
 
-    from ..transforms import SingleAutoLevel16to8
+    from ..loader import SingleAutoLevel16to8
 
     post_transforms = []
     if not RGB: