T5 with past ONNX export (#13014)

T5 with past ONNX export, and more explicit past_key_values inputs and outputs names for ONNX model

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

tests/test_onnx_v2.py

@@ -34,11 +34,7 @@ from transformers.onnx import (
     validate_model_outputs,
 )
 from transformers.onnx.config import DEFAULT_ONNX_OPSET, OnnxConfigWithPast
-from transformers.onnx.utils import (
-    compute_effective_axis_dimension,
-    compute_serialized_parameters_size,
-    flatten_output_collection_property,
-)
+from transformers.onnx.utils import compute_effective_axis_dimension, compute_serialized_parameters_size
 from transformers.testing_utils import require_onnx, require_torch, slow
 
 
@@ -95,7 +91,7 @@ class OnnxUtilsTestCaseV2(TestCase):
         ONNX exporter will export nested collections as ${collection_name}.${level_idx_0}.${level_idx_1}...${idx_n}
         """
         self.assertEqual(
-            flatten_output_collection_property("past_key", [[0], [1], [2]]),
+            OnnxConfig.flatten_output_collection_property("past_key", [[0], [1], [2]]),
             {
                 "past_key.0": 0,
                 "past_key.1": 1,