Fix type annotation to allow ref model to be null

[niansong1996]

What does this PR do? Please describe:
Current type annotation for reference_model in DpoFinetuneConfig would give a structure error when setting reference_model=null and use precomputed reference scores instead.

Does your PR introduce any breaking changes? If yes, please list them:
Small change, N/A

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)

[cbalioglu]

LGTM! Thanks for the fix @niansong1996!