[Feature]: Re-index existing photos with machine learning

[dorianim]

Feature detail

I recently got the machine-learning container to work thanks to the guide over here https://github.com/immich-app/immich/discussions/300 .

Now, I'd like all the pictures I added before getting it work to be indexed. This feature might also be helpful in the future, should there ever be changes to the machine learning which require re-indexing.

Platform

Server