Add MMR methods to chroma (#2148)

Hi, I added MMR similar to faais and milvus to chroma. Please let me
know what you think.

langchain/vectorstores/chroma.py

@@ -5,9 +5,12 @@ import logging
 import uuid
 from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Optional, Tuple
 
+import numpy as np
+
 from langchain.docstore.document import Document
 from langchain.embeddings.base import Embeddings
 from langchain.vectorstores.base import VectorStore
+from langchain.vectorstores.utils import maximal_marginal_relevance
 
 if TYPE_CHECKING:
     import chromadb
@@ -182,6 +185,69 @@ class Chroma(VectorStore):
 
         return _results_to_docs_and_scores(results)
 
+    def max_marginal_relevance_search_by_vector(
+        self,
+        embedding: List[float],
+        k: int = 4,
+        fetch_k: int = 20,
+        filter: Optional[Dict[str, str]] = None,
+    ) -> List[Document]:
+        """Return docs selected using the maximal marginal relevance.
+        Maximal marginal relevance optimizes for similarity to query AND diversity
+        among selected documents.
+        Args:
+            embedding: Embedding to look up documents similar to.
+            k: Number of Documents to return. Defaults to 4.
+            fetch_k: Number of Documents to fetch to pass to MMR algorithm.
+            filter (Optional[Dict[str, str]]): Filter by metadata. Defaults to None.
+        Returns:
+            List of Documents selected by maximal marginal relevance.
+        """
+
+        results = self._collection.query(
+            query_embeddings=embedding,
+            n_results=fetch_k,
+            where=filter,
+            include=["metadatas", "documents", "distances", "embeddings"],
+        )
+        mmr_selected = maximal_marginal_relevance(
+            np.array(embedding, dtype=np.float32), results["embeddings"][0], k=k
+        )
+
+        candidates = _results_to_docs(results)
+
+        selected_results = [r for i, r in enumerate(candidates) if i in mmr_selected]
+        return selected_results
+
+    def max_marginal_relevance_search(
+        self,
+        query: str,
+        k: int = 4,
+        fetch_k: int = 20,
+        filter: Optional[Dict[str, str]] = None,
+    ) -> List[Document]:
+        """Return docs selected using the maximal marginal relevance.
+        Maximal marginal relevance optimizes for similarity to query AND diversity
+        among selected documents.
+        Args:
+            query: Text to look up documents similar to.
+            k: Number of Documents to return. Defaults to 4.
+            fetch_k: Number of Documents to fetch to pass to MMR algorithm.
+            filter (Optional[Dict[str, str]]): Filter by metadata. Defaults to None.
+        Returns:
+            List of Documents selected by maximal marginal relevance.
+        """
+        if self._embedding_function is None:
+            raise ValueError(
+                "For MMR search, you must specify an embedding function on" "creation."
+            )
+
+        embedding = self._embedding_function.embed_query(query)
+        docs = self.max_marginal_relevance_search_by_vector(
+            embedding, k, fetch_k, filter
+        )
+        return docs
+
     def delete_collection(self) -> None:
         """Delete the collection."""
         self._client.delete_collection(self._collection.name)