Fix ModelOutput instantiation form dictionaries (#13067)

* Fix ModelOutput instantiation form dictionaries

* Style

src/transformers/file_utils.py

@@ -1850,6 +1850,10 @@ class ModelOutput(OrderedDict):
         other_fields_are_none = all(getattr(self, field.name) is None for field in class_fields[1:])
 
         if other_fields_are_none and not is_tensor(first_field):
+            if isinstance(first_field, dict):
+                iterator = first_field.items()
+                first_field_iterator = True
+            else:
                 try:
                     iterator = iter(first_field)
                     first_field_iterator = True

tests/test_model_output.py

@@ -101,3 +101,9 @@ class ModelOutputTester(unittest.TestCase):
         x["a"] = 10
         self.assertEqual(x.a, 10)
         self.assertEqual(x["a"], 10)
+
+    def test_instantiate_from_dict(self):
+        x = ModelOutputTest({"a": 30, "b": 10})
+        self.assertEqual(list(x.keys()), ["a", "b"])
+        self.assertEqual(x.a, 30)
+        self.assertEqual(x.b, 10)