faiss document store should save it's configuration alongside the index

[mathislucka]

Is your feature request related to a problem? Please describe.
As discussed in #1337 (comment) the FAISS document store should save it's configuration when the index is stored on disk. Otherwise, an end-user will have to store the config somewhere else or they'll have to remember the configuration. Also, the load method currently does not take any configuration options which means that indexes loaded from disk will always use the default configuration.

Describe the solution you'd like
When calling the save method a json (?) file is saved which contains the current configuration of the index. The load method will automatically look for a config-file which is located in the same directory as the index file. If a config file is missing, the index will be initialized with the default config (thus keeping backwards compatibility).

Describe alternatives you've considered
None

[julian-risch]

Hi @mathislucka thank you for creating this issue based on the discussion in #1337 We will discuss internally whether we can tackle this issue in one of our next sprints. If you or somebody else are/is interested in contributing here, feel free to create a work-in-progress pull request. 🙂

[tholor]

One more thing we should tackle here: When loading the DocumentStore we should assert that the number of docs in SQL match the number of docs in FAISS

[ZanSara]

Hey @mathislucka, we improved the save/load API of FAISSDocumentStore to use a small configuration file in #1366. It also implements the check mentioned by @tholor. Check it out! I'm closing the issue now, please re-open if needed.