Initial support for symbolic tracing with torch.fx allowing dynamic axes (#13579)

* Symbolic trace dynamic axes support for BERT like models (albert, bert, distilbert, mobilebert, electra, megatron-bert)
* Sanity checks before tracing that make sure the model to trace is supported
* Adapted to PyTorch 1.9

Co-authored-by: Michael Benayoun <michael@huggingface.co>

tests/test_modeling_mobilebert.py

@@ -270,6 +270,7 @@ class MobileBertModelTest(ModelTesterMixin, unittest.TestCase):
         else ()
     )
     fx_ready_model_classes = all_model_classes
+    fx_dynamic_ready_model_classes = all_model_classes
     test_sequence_classification_problem_types = True
 
     # special case for ForPreTraining model