Fixed failing Bart-Model Integration Tests

[Sai-Suraj-27]

What does this PR do?

Fixes these failing Bart-Model Integration Tests. As mentioned in the V5 Migration guide: The encode_plus method is deprecated in favor of the single __call__ method.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@ArthurZucker @Rocketknight1

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: bart