[config.data.nih_cxr14.datamodule] Fix loading of RGBA-encoded images (closes #51)

src/ptbench/config/data/nih_cxr14/datamodule.py

@@ -89,8 +89,10 @@ class RawDataLoader(_BaseRawDataLoader):
                 basename,
             )
 
-        # N.B.: NIH CXR-14 images are encoded as color PNGs
+        # N.B.: some NIH CXR-14 images are encoded as color PNGs with an alpha
+        # channel.  Most, are grayscale PNGs
         image = PIL.Image.open(os.path.join(self.datadir, file_path))
+        image = image.convert("L")  # required for some images
         tensor = to_tensor(image)
 
         # use the code below to view generated images