Is your feature request related to a problem? Please describe.
cuML will need to serialize/deserialize the ivf_flat::index (see rapidsai/cuml#4743). Some other downstream libraries might want to do the same thing.

It would be easier if RAFT provides methods to save / restore the state from simple byte strings.

Describe the solution you'd like

• Provide serialize method that return host byte string with the index state.
• Provide a deserialize method that creates an index from a byte string.

Describe alternatives you've considered
The new IVF flat code #747 provides accessors for the index state, so it is possible for each library to roll out their preferred serialization method. This requires downstream library developers to understand (part of the) implementation details of the index. It would be easier if RAFT provides the serialization method.