1. After adding get_fold_generator functionality for time series, compare_models is breaking (not returning any models). This was not caught in CI since the test for compare_models was missing.
2. Also fixed functional API for blend_models which was failing earlier due to incorrect default metric.

Describe the changes you've made

Fixed the issue mentioned above and added unit test for compare_models.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, local variables)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Unit test has been added

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.