Add ONNX support for Blenderbot and BlenderbotSmall (#15875)

* Add ONNX support for Blenderbot

* Add BlenderbotSmall ONNX configuration

* Update serialization table

tests/onnx/test_onnx_v2.py

@@ -194,6 +194,8 @@ PYTORCH_EXPORT_SEQ2SEQ_WITH_PAST_MODELS = {
     ("t5", "t5-small"),
     ("marian", "Helsinki-NLP/opus-mt-en-de"),
     ("m2m-100", "facebook/m2m100_418M"),
+    ("blenderbot-small", "facebook/blenderbot_small-90M"),
+    ("blenderbot", "facebook/blenderbot-400M-distill"),
 }
 
 # TODO(lewtun): Include the same model types in `PYTORCH_EXPORT_MODELS` once TensorFlow has parity with the PyTorch model implementations.