Feature/compute cometkiwi metric

[tanhaow]

Associated Issue(s): resolves #52

Changes in this PR

Include all key changes in this pull request

• Added compute_cometkiwi() function to metrics.py using the Unbabel/wmt22-cometkiwi-da model
• Integrated CometKiwi metric into evaluate_corpus.py script; CSV output now includes cometkiwi column alongside chrf and comet
• Added HuggingFace authentication error handling with helpful user guidance for the gated HuggingFace model (--> or should we move this to DeveloperNotes instead?)

Notes

• First run will download ~2GB model and cache it for subsequent use (Added model caching via LOADED_METRICS dictionary to reuse loaded models across evaluations)

Reviewer Checklist

Include discrete checks that should be done by the reviewer beyond looking through
code and/or file changes. Note that this check list will correspond to tasks within
the PR overview page.

• Verify compute_cometkiwi() function signature matches pattern (takes tr_text and src_text, returns float)
• Check that evaluate_corpus.py correctly computes and writes CometKiwi scores to CSV