add option to run `vf-eval` with prepared completions or by providing ground truth

[rasdani]

add option to run vf-eval in "offline" mode with prepared completions or by providing ground truth.
helps with testing and avoids inference, if not necessary.

"answer" can already be provided by dataset, just pass it through to vf-eval.

[lakshyaag]

To confirm, this would involve loading a completions file from a specified path, and then skipping over the inference block, jumping straight to comparing completions to answer via the rubrics. Is the understanding appropriate?