fix(ui): edit variant for main models only

[psychedelicious]

Summary

only main models have a variant field

Related Issues / Discussions

Closes #6444

QA Instructions

• Edit a LoRA. There should be no variant field and you should be able to change the base model.

Merge Plan

n/a

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)