Add missing Maskformer dataclass decorator, add dataclass check in ModelOutput for subclasses (#25638)

* Add @dataclass to MaskFormerPixelDecoderOutput

* Add dataclass check if subclass of ModelOutout

* Use unittest assertRaises rather than pytest per contribution doc

* Update src/transformers/utils/generic.py per suggested change

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

---------

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

tests/utils/test_model_output.py

@@ -143,3 +143,23 @@ class ModelOutputTester(unittest.TestCase):
 
         unflattened_x = torch.utils._pytree.tree_unflatten(actual_flat_outs, actual_tree_spec)
         self.assertEqual(x, unflattened_x)
+
+
+class ModelOutputTestNoDataclass(ModelOutput):
+    """Invalid test subclass of ModelOutput where @dataclass decorator is not used"""
+
+    a: float
+    b: Optional[float] = None
+    c: Optional[float] = None
+
+
+class ModelOutputSubclassTester(unittest.TestCase):
+    def test_direct_model_output(self):
+        # Check that direct usage of ModelOutput instantiates without errors
+        ModelOutput({"a": 1.1})
+
+    def test_subclass_no_dataclass(self):
+        # Check that a subclass of ModelOutput without @dataclass is invalid
+        # A valid subclass is inherently tested other unit tests above.
+        with self.assertRaises(TypeError):
+            ModelOutputTestNoDataclass(a=1.1, b=2.2, c=3.3)