Fix ModelOutput instantiation when there is only one tuple (#20416)

src/transformers/utils/generic.py

@@ -227,12 +227,20 @@ class ModelOutput(OrderedDict):
             # if we provided an iterator as first field and the iterator is a (key, value) iterator
             # set the associated fields
             if first_field_iterator:
-                for element in iterator:
+                for idx, element in enumerate(iterator):
                     if (
                         not isinstance(element, (list, tuple))
                         or not len(element) == 2
                         or not isinstance(element[0], str)
                     ):
+                        if idx == 0:
+                            # If we do not have an iterator of key/values, set it as attribute
+                            self[class_fields[0].name] = first_field
+                        else:
+                            # If we have a mixed iterator, raise an error
+                            raise ValueError(
+                                f"Cannot set key/value for {element}. It needs to be a tuple (key, value)."
+                            )
                         break
                     setattr(self, element[0], element[1])
                     if element[1] is not None:

tests/utils/test_model_output.py

@@ -107,3 +107,16 @@ class ModelOutputTester(unittest.TestCase):
         self.assertEqual(list(x.keys()), ["a", "b"])
         self.assertEqual(x.a, 30)
         self.assertEqual(x.b, 10)
+
+    def test_instantiate_from_iterator(self):
+        x = ModelOutputTest([("a", 30), ("b", 10)])
+        self.assertEqual(list(x.keys()), ["a", "b"])
+        self.assertEqual(x.a, 30)
+        self.assertEqual(x.b, 10)
+
+        with self.assertRaises(ValueError):
+            _ = ModelOutputTest([("a", 30), (10, 10)])
+
+        x = ModelOutputTest(a=(30, 30))
+        self.assertEqual(list(x.keys()), ["a"])
+        self.assertEqual(x.a, (30, 30))