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/xlnet/test_modeling_xlnet.py

@@ -526,6 +526,7 @@ class XLNetModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase)
     all_generative_model_classes = (
         (XLNetLMHeadModel,) if is_torch_available() else ()
     )  # TODO (PVP): Check other models whether language generation is also applicable
+    fx_compatible = False
     test_pruning = False
 
     # XLNet has 2 QA models -> need to manually set the correct labels for one of them here