Fix: zephyr reranker returns list when single result expected

[astappiev]

Issue

When using Zephyr from RankLLM, it returns a list of results (because it calls rerank_batch with a single item).

Naturally, it's not expected behaviour, and the rerankers crash with an error

list does not have attribute 'candidates'

Changes

We check if we got a list, and if so, unwrap it.

Sending a PR to upstream might be more difficult because it will be a breaking change there.

Testing

Zephyr reranker from RankLLM works after fixing this issue.