Be more consistent with OOP/functional programming

[RobotSail]

There seems to currently be a mixture of OOP principles as well as functional principles throughout the codebase. Some of these seem like they can be combined.

For example, MMLUEvaluator implements all common functionality in a base abstract class which its variants then inherit.

However; MTBench seems to implement some base methods in the class, but also call out to generate_answers functionally, which seems counterintuitive to intended functionality.

eval/src/instructlab/eval/mt_bench_answers.py

Line 97 in f4db222

def generate_answers(

We should combine this into the existing abstract class or make everuthing functional.