FAISS should save/load its index_to_id map

[xloem]

The save_local, load_local member functions in vectorstore.faiss seem firstly to need to read/write the index_to_id map as well as the main index file. This would then fully separate the storage concerns between the Docstore and the VectorStore.

As it is now, the index_to_id map is an additional component the user must separately serialize/deserialize to reload an FAISS index associated with a Docstore.

[ShreyJ1729]

Check out #789 and #880.

[xloem]

Thanks for your work on this. It looks like the code currently in tree is broken and any change could be an improvement. Closing this as it is a duplicate.