stop using global config for non-parallel generation

[mivanit]

This was just kind of a bad decision on my part. To simplify/optimize parallel generation, in MazeDataset.generate() we:

• run _maze_gen_init_worker(config: MazeDatasetConfig), which initializes a global _GLOBAL_WORKER_CONFIG
• imap the _generate_maze_helper(index: int) function

this is done to avoid passing the whole config around, and it might not be that bad of an idea for parallel generation, but this is done even when the generation is done on a single thread.

Two options:

• if passing the config does not have that much of an overhead, then just pass it
• don't use a global worker init for the non-parallel generation
• use functools.partial instead (I think I originally tried to use a lambda but that of course does not pickle) and remove the need for a global worker config

I'm partial to the last one, since it feels cleanest.