fix: Add bm25s as a model for retrieval

[isaac-chung]

Addresses #990.

With Stopwords=None and the Snowball Stemmer, I can reproduce NFCorpus from the paper's table 3 at 32.0 NDCG@10.

Checklist

• Move results to the results repo
• Run tests locally to make sure nothing is broken using make test.
• Run the formatter to format the code using make lint.

Adding a model checklist

• I have filled out the ModelMeta object to the extent possible
• I have ensured that my model can be loaded using
  • mteb.get_model(model_name, revision_id) and
  • mteb.get_model_meta(model_name, revision_id)
• I have tested the implementation works on a representative set of tasks.

Co-authored by: @malteos github@i.mieo.de

[isaac-chung]

@Muennighoff running the rest of the English benchmark now. Will push when I have all results.

[isaac-chung]

Before merging I'd like to add the co-author line in the squashed commit to highlight the foundation work done by @malteos 🚀

[malteos]

Thanks for making the PR!