BART: disable returning kv states since there exists an on device cache

[kundaMwiza]

What does this PR do?

Given that the kv cache is on device, there is no need to return past_key_values. However, this would require overriding the forward methods of BartDecoder and BartDecoderLayer

Fixes # (issue)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

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