Fx support for multiple model architectures (#17393)

* Support for Bart and LayoutLM, and partial support for XLNet

* Support for mbart

* A lot of new models supported

* Support for other models

* LayoutLM fix

* Use strings instead of classes

tests/models/layoutlm/test_modeling_layoutlm.py

@@ -215,6 +215,7 @@ class LayoutLMModelTest(ModelTesterMixin, unittest.TestCase):
         if is_torch_available()
         else None
     )
+    fx_compatible = True
 
     def setUp(self):
         self.model_tester = LayoutLMModelTester(self)