Make square centre-padding a model transform

This MR fixes issues #23 (closed) and #39 (closed), by:

• Removing centre-cropping from all databases raw-data loaders
• Creating a SquareCenterPad transform that operates on tensors
• Adding that transform to all relevant models (Pasa, Alexnet, Densenet)

src/mednet/models/alexnet.py

@@ -15,7 +15,7 @@ import torchvision.transforms
 
 from ..data.typing import TransformSequence
 from .separate import separate
-from .transforms import RGB
+from .transforms import RGB, SquareCenterPad
 from .typing import Checkpoint
 
 logger = logging.getLogger(__name__)
@@ -74,6 +74,7 @@ class Alexnet(pl.LightningModule):
         self.num_classes = num_classes
 
         self.model_transforms = [
+            SquareCenterPad(),
             torchvision.transforms.Resize(512, antialias=True),
             RGB(),
         ]