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Cross-Encoder Re-Ranking

A second approach to re-ranking is to use a cross-encoder. Our semantic search embeddings were created with a bi-encoder, which embeds queries and documents separately so that we can use cosine similarity to get a relevance score.

A cross-encoder does the same thing, but embeds the query and document as a single input to produce a relevance score directly. The output is the score – no need to compute cosine similarity.

Cross-encoders see the full context of how the query and document interact, so they can catch subtle relationships that bi-encoders miss.

Re-Ranking Speed

One big advantage of cross-encoders is that they are much faster and cheaper than LLMs, as they do not require the same level of computational resources. Instead of using a tool with all the capabilities of an LLM, this type of re-ranker can only do one thing: take in a query and document pair and output a relevance score.

In other words, it's a regression model.

Fine-Tuning Cross-Encoders

The second major advantage of cross-encoders is that they can be fine-tuned on your specific domain relatively easily. You can train them on your own query-document pairs to learn the exact relevance patterns for your use case, and they are cheaper and faster than LLMs.

We won't be doing any fine-tuning in this course, but it's good to know about!

A fast way to get up and running with a re-ranker is to use the Cohere API or one of its competitors. Typically these APIs use cross-encoders.

Assignment

Implement a cross-encoder re-ranker for RRF search.

Add a new option to the --rerank-method argument. Now it can be "individual", "batch", or "cross_encoder".
Again, gather 5 times as many results as the limit specified if a --rerank-method is set.
If the "cross_encoder" re-rank method is provided, after doing the initial RRF search:
- Create a new list of "pairs" lists, where the first element is the query, and the second element is the document, in this stringified format:
```
pairs.append([query, f"{doc.get('title', '')} - {doc.get('document', '')}"])
```
- From the same sentence_transformers library as before, import the CrossEncoder class.
- Create a new CrossEncoder instance (just do this once per query, not for each document) using the cross-encoder/ms-marco-TinyBERT-L2-v2 model.
```
cross_encoder = CrossEncoder("cross-encoder/ms-marco-TinyBERT-L2-v2")
```
  If you get an error about your GPU or hardware acceleration, load the cross-encoder on your CPU instead by passing device="cpu".
- Use the CrossEncoder to compute scores for all the pairs:
```
# `predict` returns a list of numbers, one for each pair
scores = cross_encoder.predict(pairs)
```
Sort the results by the new score in descending order.

Print the results after truncating to the limit, preserving the RRF score and the new cross-encoder score in this format:

Re-ranking top 25 results using cross_encoder method...
Reciprocal Rank Fusion Results for 'family movie about bears in the woods' (k=60):

1. Care Bears Movie II: A New Generation
   Cross Encoder Score: 3.314
   RRF Score: 0.028
   BM25 Rank: 1, Semantic Rank: 26
   Aboard their boat, a yellow bear and a purple horse look after the baby animals known as the Care Be...

2. A Bear for Punishment
   Cross Encoder Score: -1.001
   RRF Score: 0.020
   BM25 Rank: 41, Semantic Rank: 35
   The film begins with the bear family sleeping peacefully at home, when suddenly, the alarms of dozen...

3. The Country Bears
   Cross Encoder Score: -1.276
   RRF Score: 0.023
   BM25 Rank: 25, Semantic Rank: 32
   Beary Barrington is a young bear who has been raised by a human family and struggles with his identi...

Submit the CLI tests.