Add mt5 onnx config (#18394)

* update features

* MT5OnnxConfig added with updated with tests and docs

* fix imports

* fix onnc_config_cls for mt5

Co-authored-by: Thomas Chaigneau <thomas.deeptools.ai>

tests/onnx/test_onnx_v2.py

@@ -224,6 +224,7 @@ PYTORCH_EXPORT_SEQ2SEQ_WITH_PAST_MODELS = {
     ("mbart", "sshleifer/tiny-mbart"),
     ("t5", "t5-small"),
     ("marian", "Helsinki-NLP/opus-mt-en-de"),
+    ("mt5", "google/mt5-base"),
     ("m2m-100", "facebook/m2m100_418M"),
     ("blenderbot-small", "facebook/blenderbot_small-90M"),
     ("blenderbot", "facebook/blenderbot-400M-distill"),