community[patch]: Added missing from_documents method to KNNRetriever. (#18411)

- Description: Added missing `from_documents` method to `KNNRetriever`,
providing the ability to supply metadata to LangChain `Document`s, and
to give it parity to the other retrievers, which do have
`from_documents`.
- Issue: None
- Dependencies: None
- Twitter handle: None

Co-authored-by: Victor Adan <vadan@netroadshow.com>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/community/tests/unit_tests/retrievers/test_knn.py

@@ -1,3 +1,5 @@
+from langchain_core.documents import Document
+
 from langchain_community.embeddings import FakeEmbeddings
 from langchain_community.retrievers.knn import KNNRetriever
 
@@ -9,3 +11,19 @@ class TestKNNRetriever:
             texts=input_texts, embeddings=FakeEmbeddings(size=100)
         )
         assert len(knn_retriever.texts) == 3
+
+    def test_from_documents(self) -> None:
+        input_docs = [
+            Document(page_content="I have a pen.", metadata={"page": 1}),
+            Document(page_content="Do you have a pen?", metadata={"page": 2}),
+            Document(page_content="I have a bag.", metadata={"page": 3}),
+        ]
+        knn_retriever = KNNRetriever.from_documents(
+            documents=input_docs, embeddings=FakeEmbeddings(size=100)
+        )
+        assert knn_retriever.texts == [
+            "I have a pen.",
+            "Do you have a pen?",
+            "I have a bag.",
+        ]
+        assert knn_retriever.metadatas == [{"page": 1}, {"page": 2}, {"page": 3}]