Some databases have file structures very flat (with more than 30k files in a directory). This is not good for the Idiap file structure and can let our I/O super slow. We should implement a hash function in `CheckpointWrapper.make_path` that generates directory names given `sample.key` to limit the number of files in a directory to 1000 files. ping @lcolbois (this touches experiments with IJB-C)