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/pegasus/test_modeling_pegasus.py

@@ -229,6 +229,7 @@ class PegasusModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCas
     all_model_classes = (PegasusModel, PegasusForConditionalGeneration) if is_torch_available() else ()
     all_generative_model_classes = (PegasusForConditionalGeneration,) if is_torch_available() else ()
     is_encoder_decoder = True
+    fx_compatible = True
     test_resize_position_embeddings = True
     test_pruning = False
     test_missing_keys = False