[fix] serial training during pytest; fix ucir training errors

[Moenupa]

What does this PR do?

Fixes:

• serial training during pytest
• UCIR index Error as default K=2, seen_class=1.

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?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?