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/bart/test_modeling_bart.py

@@ -413,6 +413,7 @@ class BartModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase):
     )
     all_generative_model_classes = (BartForConditionalGeneration,) if is_torch_available() else ()
     is_encoder_decoder = True
+    fx_compatible = True
     test_pruning = False
     test_missing_keys = False
 
@@ -1386,6 +1387,7 @@ class BartStandaloneDecoderModelTester:
 class BartStandaloneDecoderModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase):
     all_model_classes = (BartDecoder, BartForCausalLM) if is_torch_available() else ()
     all_generative_model_classes = (BartForCausalLM,) if is_torch_available() else ()
+    fx_comptatible = True
     test_pruning = False
     is_encoder_decoder = False