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/m2m_100/test_modeling_m2m_100.py

@@ -231,6 +231,7 @@ class M2M100ModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase
     )
     all_generative_model_classes = (M2M100ForConditionalGeneration,) if is_torch_available() else ()
     is_encoder_decoder = True
+    fx_compatible = True
     test_pruning = False
     test_missing_keys = False