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/mbart/test_modeling_mbart.py

@@ -224,6 +224,7 @@ class MBartModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase)
     )
     all_generative_model_classes = (MBartForConditionalGeneration,) if is_torch_available() else ()
     is_encoder_decoder = True
+    fx_compatible = True
     test_pruning = False
     test_missing_keys = False