Fixes wrong label in FilePathDataset (#94)

* Fixes #93

Instead of labels, file names were used for making label-to-class map.

* Fix label mapping in FilePathDataset __getitem__

* Cleanup label to class mapping code.

flash/vision/classification/data.py

@@ -56,7 +56,7 @@ def __init__(
         self.transform = transform
         self.loader = loader
         if self.has_labels:
-            self.label_to_class_mapping = {v: k for k, v in enumerate(list(sorted(list(set(self.fnames)))))}
+            self.label_to_class_mapping = dict(map(reversed, enumerate(sorted(set(self.labels)))))
 
     @property
     def has_labels(self) -> bool:
@@ -70,7 +70,8 @@ def __getitem__(self, index: int) -> Tuple[Any, Optional[int]]:
         img = self.loader(filename)
         label = None
         if self.has_labels:
-            label = self.label_to_class_mapping[filename]
+            label = self.labels[index]
+            label = self.label_to_class_mapping[label]
         return img, label