add with score option for max marginal relevance (#6867)

### Adding the functionality to return the scores with retrieved
documents when using the max marginal relevance
- Description: Add the method
`max_marginal_relevance_search_with_score_by_vector` to the FAISS
wrapper. Functionality operates the same as
`similarity_search_with_score_by_vector` except for using the max
marginal relevance retrieval framework like is used in the
`max_marginal_relevance_search_by_vector` method.
  - Dependencies: None
  - Tag maintainer: @rlancemartin @eyurtsev 
  - Twitter handle: @RianDolphin

---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/vectorstores/faiss.py

@@ -37,7 +37,7 @@ def dependable_faiss_import(no_avx2: Optional[bool] = None) -> Any:
         else:
             import faiss
     except ImportError:
-        raise ValueError(
+        raise ImportError(
             "Could not import faiss python package. "
             "Please install it with `pip install faiss` "
             "or `pip install faiss-cpu` (depending on Python version)."
@@ -321,16 +321,17 @@ class FAISS(VectorStore):
         )
         return [doc for doc, _ in docs_and_scores]
 
-    def max_marginal_relevance_search_by_vector(
+    def max_marginal_relevance_search_with_score_by_vector(
         self,
         embedding: List[float],
+        *,
         k: int = 4,
         fetch_k: int = 20,
         lambda_mult: float = 0.5,
         filter: Optional[Dict[str, Any]] = None,
-        **kwargs: Any,
-    ) -> List[Document]:
-        """Return docs selected using the maximal marginal relevance.
+    ) -> List[Tuple[Document, float]]:
+        """Return docs and their similarity scores selected using the maximal marginal
+            relevance.
 
         Maximal marginal relevance optimizes for similarity to query AND diversity
         among selected documents.
@@ -345,9 +346,10 @@ class FAISS(VectorStore):
                         to maximum diversity and 1 to minimum diversity.
                         Defaults to 0.5.
         Returns:
-            List of Documents selected by maximal marginal relevance.
+            List of Documents and similarity scores selected by maximal marginal
+                relevance and score for each.
         """
-        _, indices = self.index.search(
+        scores, indices = self.index.search(
             np.array([embedding], dtype=np.float32),
             fetch_k if filter is None else fetch_k * 2,
         )
@@ -373,8 +375,9 @@ class FAISS(VectorStore):
             lambda_mult=lambda_mult,
         )
         selected_indices = [indices[0][i] for i in mmr_selected]
-        docs = []
-        for i in selected_indices:
+        selected_scores = [scores[0][i] for i in mmr_selected]
+        docs_and_scores = []
+        for i, score in zip(selected_indices, selected_scores):
             if i == -1:
                 # This happens when not enough docs are returned.
                 continue
@@ -382,8 +385,39 @@ class FAISS(VectorStore):
             doc = self.docstore.search(_id)
             if not isinstance(doc, Document):
                 raise ValueError(f"Could not find document for id {_id}, got {doc}")
-            docs.append(doc)
-        return docs
+            docs_and_scores.append((doc, score))
+        return docs_and_scores
+
+    def max_marginal_relevance_search_by_vector(
+        self,
+        embedding: List[float],
+        k: int = 4,
+        fetch_k: int = 20,
+        lambda_mult: float = 0.5,
+        filter: Optional[Dict[str, Any]] = None,
+        **kwargs: Any,
+    ) -> 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 before filtering to
+                     pass to MMR algorithm.
+            lambda_mult: Number between 0 and 1 that determines the degree
+                        of diversity among the results with 0 corresponding
+                        to maximum diversity and 1 to minimum diversity.
+                        Defaults to 0.5.
+        Returns:
+            List of Documents selected by maximal marginal relevance.
+        """
+        docs_and_scores = self.max_marginal_relevance_search_with_score_by_vector(
+            embedding, k=k, fetch_k=fetch_k, lambda_mult=lambda_mult, filter=filter
+        )
+        return [doc for doc, _ in docs_and_scores]
 
     def max_marginal_relevance_search(
         self,
@@ -414,8 +448,8 @@ class FAISS(VectorStore):
         embedding = self.embedding_function(query)
         docs = self.max_marginal_relevance_search_by_vector(
             embedding,
-            k,
-            fetch_k,
+            k=k,
+            fetch_k=fetch_k,
             lambda_mult=lambda_mult,
             filter=filter,
             **kwargs,

tests/integration_tests/vectorstores/test_faiss.py

@@ -46,9 +46,19 @@ def test_faiss_vector_sim() -> None:
     output = docsearch.similarity_search_by_vector(query_vec, k=1)
     assert output == [Document(page_content="foo")]
 
+
+def test_faiss_mmr() -> None:
+    texts = ["foo", "foo", "fou", "foy"]
+    docsearch = FAISS.from_texts(texts, FakeEmbeddings())
+    query_vec = FakeEmbeddings().embed_query(text="foo")
     # make sure we can have k > docstore size
-    output = docsearch.max_marginal_relevance_search_by_vector(query_vec, k=10)
+    output = docsearch.max_marginal_relevance_search_with_score_by_vector(
+        query_vec, k=10, lambda_mult=0.1
+    )
     assert len(output) == len(texts)
+    assert output[0][0] == Document(page_content="foo")
+    assert output[0][1] == 0.0
+    assert output[1][0] != Document(page_content="foo")
 
 
 def test_faiss_with_metadatas() -> None: