infra: add test for ensemble retriever to ensure multiple retrievers (#8401)

Add tests to ensemble retriever to ensure it works with combination of
multiple retrievers

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/tests/unit_tests/retrievers/test_ensemble.py

@@ -1,6 +1,8 @@
 import pytest
 from langchain_core.documents import Document
 
+from langchain.embeddings import FakeEmbeddings
+from langchain.retrievers import KNNRetriever, TFIDFRetriever
 from langchain.retrievers.bm25 import BM25Retriever
 from langchain.retrievers.ensemble import EnsembleRetriever
 
@@ -40,3 +42,38 @@ def test_weighted_reciprocal_rank() -> None:
     result = ensemble_retriever.weighted_reciprocal_rank([[doc1, doc2], [doc2, doc1]])
     assert result[0].page_content == "1"
     assert result[1].page_content == "2"
+
+
+@pytest.mark.requires("rank_bm25", "sklearn")
+def test_ensemble_retriever_get_relevant_docs_with_multiple_retrievers() -> None:
+    doc_list_a = [
+        "I like apples",
+        "I like oranges",
+        "Apples and oranges are fruits",
+    ]
+    doc_list_b = [
+        "I like melons",
+        "I like pineapples",
+        "Melons and pineapples are fruits",
+    ]
+    doc_list_c = [
+        "I like avocados",
+        "I like strawberries",
+        "Avocados and strawberries are fruits",
+    ]
+
+    dummy_retriever = BM25Retriever.from_texts(doc_list_a)
+    dummy_retriever.k = 1
+    tfidf_retriever = TFIDFRetriever.from_texts(texts=doc_list_b)
+    tfidf_retriever.k = 1
+    knn_retriever = KNNRetriever.from_texts(
+        texts=doc_list_c, embeddings=FakeEmbeddings(size=100)
+    )
+    knn_retriever.k = 1
+
+    ensemble_retriever = EnsembleRetriever(
+        retrievers=[dummy_retriever, tfidf_retriever, knn_retriever],
+        weights=[0.6, 0.3, 0.1],
+    )
+    docs = ensemble_retriever.get_relevant_documents("I like apples")
+    assert len(docs) == 3