Check TF ops for ONNX compliance (#10025)

* Add check-ops script

* Finish to implement check_tf_ops and start the test

* Make the test mandatory only for BERT

* Update tf_ops folder

* Remove useless classes

* Add the ONNX test for GPT2 and BART

* Add a onnxruntime slow test + better opset flexibility

* Fix test + apply style

* fix tests

* Switch min opset from 12 to 10

* Update src/transformers/file_utils.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Fix GPT2

* Remove extra shape_list usage

* Fix GPT2

* Address Morgan's comments

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

tests/test_modeling_tf_gpt2.py

@@ -333,6 +333,8 @@ class TFGPT2ModelTest(TFModelTesterMixin, unittest.TestCase):
     )
     all_generative_model_classes = (TFGPT2LMHeadModel,) if is_tf_available() else ()
     test_head_masking = False
+    test_onnx = True
+    onnx_min_opset = 10
 
     def setUp(self):
         self.model_tester = TFGPT2ModelTester(self)