Implement Birch/MaxP variant

[lintool]

We should probably implement our own variant of Birch/BERT-MaxP in PyGaggle -

• Birch: https://www.aclweb.org/anthology/D19-1352/
• BERT-MaxP: https://dl.acm.org/doi/10.1145/3331184.3331303

Let's call it MonoBirchP :)

[ronakice]

Sorry, I missed this, we're already doing MaxP and sum-based aggregation in Document Ranking. Birch is just a generalized weight-based sum aggregator that linearly interpolates with the BM25 scores right?

[lintool]

Correct. This is probably lower priority because we also have https://capreolus.ai/