Add workaround for not having async vector store methods (#2733)

This allows us to use the async API for the Retrieval chains, though it is not guaranteed to be thread safe.

tests/integration_tests/vectorstores/test_chroma.py

@@ -1,4 +1,6 @@
 """Test Chroma functionality."""
+import pytest
+
 from langchain.docstore.document import Document
 from langchain.vectorstores import Chroma
 from tests.integration_tests.vectorstores.fake_embeddings import FakeEmbeddings
@@ -14,6 +16,17 @@ def test_chroma() -> None:
     assert output == [Document(page_content="foo")]
 
 
+@pytest.mark.asyncio
+async def test_chroma_async() -> None:
+    """Test end to end construction and search."""
+    texts = ["foo", "bar", "baz"]
+    docsearch = Chroma.from_texts(
+        collection_name="test_collection", texts=texts, embedding=FakeEmbeddings()
+    )
+    output = await docsearch.asimilarity_search("foo", k=1)
+    assert output == [Document(page_content="foo")]
+
+
 def test_chroma_with_metadatas() -> None:
     """Test end to end construction and search."""
     texts = ["foo", "bar", "baz"]