Chroma persistence (#1028)

This PR adds persistence to the Chroma vector store.

Users can supply a `persist_directory` with any of the `Chroma` creation
methods. If supplied, the store will be automatically persisted at that
directory.

If a user creates a new `Chroma` instance with the same persistence
directory, it will get loaded up automatically. If they use `from_texts`
or `from_documents` in this way, the documents will be loaded into the
existing store.

There is the chance of some funky behavior if the user passes a
different embedding function from the one used to create the collection
- we will make this easier in future updates. For now, we log a warning.

tests/integration_tests/vectorstores/test_chroma.py

@@ -26,3 +26,36 @@ def test_chroma_with_metadatas() -> None:
     )
     output = docsearch.similarity_search("foo", k=1)
     assert output == [Document(page_content="foo", metadata={"page": "0"})]
+
+
+def test_chroma_with_persistence() -> None:
+    """Test end to end construction and search, with persistence."""
+    chroma_persist_dir = "./tests/persist_dir"
+    collection_name = "test_collection"
+    texts = ["foo", "bar", "baz"]
+    docsearch = Chroma.from_texts(
+        collection_name=collection_name,
+        texts=texts,
+        embedding=FakeEmbeddings(),
+        persist_directory=chroma_persist_dir,
+    )
+
+    output = docsearch.similarity_search("foo", k=1)
+    assert output == [Document(page_content="foo")]
+
+    docsearch.persist()
+
+    # Get a new VectorStore from the persisted directory
+    docsearch = Chroma(
+        collection_name=collection_name,
+        embedding_function=FakeEmbeddings(),
+        persist_directory=chroma_persist_dir,
+    )
+    output = docsearch.similarity_search("foo", k=1)
+
+    # Clean up
+    docsearch.delete_collection()
+
+    # Persist doesn't need to be called again
+    # Data will be automatically persisted on object deletion
+    # Or on program exit