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