✨ feat(constants.py): add constants file for langflow interface custom types

This commit adds a new file `constants.py` to the `src/backend/langflow/interface/custom` directory. The file defines a dictionary `LANGCHAIN_BASE_TYPES` which maps string names to corresponding langflow interface custom types. This file will be used to store and access the custom types used in the langflow interface.

src/backend/langflow/interface/custom/constants.py

@@ -0,0 +1,23 @@
+from langchain import PromptTemplate
+from langchain.chains.base import Chain
+from langchain.document_loaders.base import BaseLoader
+from langchain.embeddings.base import Embeddings
+from langchain.llms.base import BaseLLM
+from langchain.schema import BaseRetriever, Document
+from langchain.text_splitter import TextSplitter
+from langchain.tools import Tool
+from langchain.vectorstores.base import VectorStore
+
+
+LANGCHAIN_BASE_TYPES = {
+    "Chain": Chain,
+    "Tool": Tool,
+    "BaseLLM": BaseLLM,
+    "PromptTemplate": PromptTemplate,
+    "BaseLoader": BaseLoader,
+    "Document": Document,
+    "TextSplitter": TextSplitter,
+    "VectorStore": VectorStore,
+    "Embeddings": Embeddings,
+    "BaseRetriever": BaseRetriever,
+}