General cleanup

[RedTachyon]

I'm going through the repository and fixing whatever minor issues I spot. For now I'm not doing anything significant, but I'll write down potentially more complex things that need to be fixed before a full release

• data_collector.py#L102 -- self._current_seed has type str | int, which seems a bit sus
• data_collectory.py#L140 -- There has to be a simpler way of writing this using defaultdict or setdefault or something like that
• data_collector.py#L66-69 -- Can't the type hints be simple callables?
• The type hint for EpisodeBuffer is meant (?) to convey that it's a nested dictionary where leaves are lists. This doesn't work because self._current_seed: int | str is one of the possible top-level values