[Bart/Marian] ignore output_attentions when invoked through AutoModel
[ ] should mimic behavior of other models
[ ] backwards compat?
this works for me:
```python
modelname='Helsinki-NLP/opus-mt-en-de'
model = MarianMTModel.from_pretrained(modelname,output_attentions=True, output_hidden_states=True)
output_tuple = model(**model.dummy_inputs)
assert len(output_tuple) == 6
```<|||||>#4388 is the real issue.