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>

src/transformers/testing_utils.py

@@ -28,6 +28,7 @@ from .file_utils import (
     is_datasets_available,
     is_faiss_available,
     is_flax_available,
+    is_onnx_available,
     is_pandas_available,
     is_scatter_available,
     is_sentencepiece_available,
@@ -160,6 +161,13 @@ def require_git_lfs(test_case):
         return test_case
 
 
+def require_onnx(test_case):
+    if not is_onnx_available():
+        return unittest.skip("test requires ONNX")(test_case)
+    else:
+        return test_case
+
+
 def require_torch(test_case):
     """
     Decorator marking a test that requires PyTorch.