Generate: pin number of beams in BART test
# What does this PR do?

A recent change added the `num_beams==1` requirement for contrastive search. One of the tests started failing because of that change -- BART has `num_beams=4` [in its config](https://huggingface.co/facebook/bart-large-cnn/blob/main/config.json#L49), so the test was now triggering beam search, and not contrastive search. This PR corrects it.

(The long-term solution is to add argument validation to detect these cases)


_The documentation is not available anymore as the PR was closed or merged._