Fix cohere api

[sanal-cohere]

Fix the use of Cohere API by using the ChatCohere class of langchain-cohere

[nejumi]

Thank you very much for the PR. I'll conduct test on it. Once confirming it's OK, it will be merged.

[sanal-cohere]

Sounds good, thank you! Let me know if you run into any issues.

[nejumi]

@sanal-cohere Sorry for having you wait so long. I reviewed your PR and found that some additional correction is needed as MT-Bench part uses separated engine (This is our fault. Sorry for our messy codebase). I will send you a PR. After your review and merging my PR, I will merge your PR.

By the way, the API version of Command R+ performed much better than the open weight version in toxicity, though it does roughly the same in other categories. Does it equip guardrails functionality which open weight one does not have? This is interesting.
https://api.wandb.ai/links/wandb-japan/quoisa1f