Fix issue in label Transform

Summary: In the construction of Vocab within label transform, the default index is set to 0. This index is returned when OOV token is given. For this transform, the default index should never be set. Otherwise, it will return default index (which is 0) for unknown labels that might get passed (Ideally it should throw error in this case because we do not know what to do when wrong label is passed for query)

Reviewed By: hudeven

Differential Revision: D32610834

fbshipit-source-id: e49385fb313929627c41fc515b6d900a6bfc3591

Author: parmeet

test/test_transforms.py

@@ -95,6 +95,9 @@ def test_labeltoindex(self):
         expected = [0, 1, 2]
         self.assertEqual(actual, expected)
 
+        with self.assertRaises(RuntimeError):
+            transform(['OOV'])
+
         transform = transforms.LabelToIndex(label_names=label_names, sort_names=True)
         actual = transform(label_names)
         expected = [2, 1, 0]

torchtext/transforms.py

@@ -130,7 +130,7 @@ def __init__(
 
         if sort_names:
             label_names = sorted(label_names)
-        self._label_vocab = Vocab(torch.classes.torchtext.Vocab(label_names, 0))
+        self._label_vocab = Vocab(torch.classes.torchtext.Vocab(label_names, None))
         self._label_names = self._label_vocab.get_itos()
 
     def forward(self, labels: Union[str, List[str]]) -> Union[int, List[int]]: