Experimental symbolic tracing feature with torch.fx for BERT, ELECTRA and T5 (#11475)

Symbolic tracing feature for BERT, ELECTRA and T5

Co-authored-by: Michael Benayoun <michael@huggingface.co>
Co-authored-by: Stas Bekman <stas@stason.org>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

tests/test_modeling_electra.py

@@ -287,6 +287,7 @@ class ElectraModelTest(ModelTesterMixin, unittest.TestCase):
         if is_torch_available()
         else ()
     )
+    fx_ready_model_classes = all_model_classes
     test_sequence_classification_problem_types = True
 
     # special case for ForPreTraining model