Add default db loader, fix import

This commits builds on DumoeDss's PR. It

- adds a default db directory name.
- adds a default db instance (Chroma).

Both points offers flexibility for users who
want to use default and users who want to
customize

Lastly, it fixes an import

embedchain/embedchain.py

@@ -11,6 +11,7 @@ from embedchain.loaders.web_page import WebPageLoader
 from embedchain.chunkers.youtube_video import YoutubeVideoChunker
 from embedchain.chunkers.pdf_file import PdfFileChunker
 from embedchain.chunkers.web_page import WebPageChunker
+from embedchain.vectordb.chroma_db import ChromaDB
 
 load_dotenv()
 
@@ -21,13 +22,15 @@ DB_DIR = os.path.join(ABS_PATH, "db")
 
 
 class EmbedChain:
-    def __init__(self, db):
+    def __init__(self, db=None):
         """
          Initializes the EmbedChain instance, sets up a vector DB client and
         creates a collection.
 
         :param db: The instance of the VectorDB subclass.
         """
+        if db is None:
+            db = ChromaDB()
         self.db_client = db.client
         self.collection = db.collection
         self.user_asks = []

embedchain/vectordb/chroma_db.py

@@ -1,15 +1,19 @@
-import os
 import chromadb
-from base_vector_db import BaseVectorDB
+import os
+
 from chromadb.utils import embedding_functions
 
+from embedchain.vectordb.base_vector_db import BaseVectorDB
+
 openai_ef = embedding_functions.OpenAIEmbeddingFunction(
     api_key=os.getenv("OPENAI_API_KEY"),
     model_name="text-embedding-ada-002"
 )
 
 class ChromaDB(BaseVectorDB):
-    def __init__(self, db_dir):
+    def __init__(self, db_dir=None):
+        if db_dir is None:
+            db_dir = "db"
         self.client_settings = chromadb.config.Settings(
             chroma_db_impl="duckdb+parquet",
             persist_directory=db_dir,