Allow user to provide output/qrel file names with dataset retrieval

[lilyjge]

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Following a discussion with @sahel-sh :

There are two modes for retrieval 1- dataset name+ retrieval method 2- requests.jsonl file which bypasses retrieval.

in mode 1, the proper qrels file is used based on the combo of the dataset name. and the other three output files are not needed, since the names for them are calculated in https://github.com/castorini/rank_llm/blob/main/src/rank_llm/rerank/reranker.py#L96C9-L96C29.
In absence of retreival method and other info in mode 2, we need to get these names from users.

This PR allows the user to pass in names for output or qrels files in mode 1, and if they're provided, they'll be used. Otherwise, fallback to the current, default values for these files (for output files, constructed file names; and for qrels, the ones mapped to the dataset).

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[sahel-sh]

thanks