adding add_documents and aadd_documents to class RedisVectorStoreRetriever (#3419)

Ran into this issue In vectorstores/redis.py when trying to use the
AutoGPT agent with redis vector store. The error I received was

`
langchain/experimental/autonomous_agents/autogpt/agent.py", line 134, in
run
    self.memory.add_documents([Document(page_content=memory_to_add)])
AttributeError: 'RedisVectorStoreRetriever' object has no attribute
'add_documents'
`

Added the needed function to the class RedisVectorStoreRetriever which
did not have the functionality like the base VectorStoreRetriever in
vectorstores/base.py that, for example, vectorstores/faiss.py has

langchain/vectorstores/redis.py

@@ -461,3 +461,13 @@ class RedisVectorStoreRetriever(BaseRetriever, BaseModel):
 
     async def aget_relevant_documents(self, query: str) -> List[Document]:
         raise NotImplementedError("RedisVectorStoreRetriever does not support async")
+
+    def add_documents(self, documents: List[Document], **kwargs: Any) -> List[str]:
+        """Add documents to vectorstore."""
+        return self.vectorstore.add_documents(documents, **kwargs)
+
+    async def aadd_documents(
+        self, documents: List[Document], **kwargs: Any
+    ) -> List[str]:
+        """Add documents to vectorstore."""
+        return await self.vectorstore.aadd_documents(documents, **kwargs)