Remove unused imports

[EliahKagan]

This removes unused imports, both in the current code (npc_gzip/, examples/, and tests/) and in the old code (original_codebase/).

Either all or nearly all unused imports are removed, depending on precisely how one defines "unused." I avoided removing imports (or making any changes) where imports are unreferenced but in some other sense used, such that removing them without making other code changes would cause breakage or alter behavior.

Info on "unused" imports that are needed and not removed (click to expand if interested)

There are only two such places.

• In the new code, npc_gzip/__init__.py, has imports but no __all__, so technically those imports are nonpublic, and since no code in that file uses them (there is no other code in that file), they are "unused". It may be better to avoid * imports and assign __all__ in that file, but I would consider such a change outside the scope of this PR.
• In the old code, eval(args.dataset)(root=args.data_dir) is used to load datasets dynamically in a way that requires them to have been imported into the module, yet the imports are "unused" in the sense that no code written in the module refers to them. It may be best to have a comment about this, or use importlib instead of an eval-based way, but such changes are not necessarily justified for the original codebase that is not under active development. Even if such changes are made, they, too, are outside the intended scope of this PR.

All other unused imports are removed.

Although major changes probably shouldn't be made to the original codebase without strong reason, it is clear from context that the unused imports in it are mostly due to other fairly recent changes (some as recent as #30).

I have tested this change, both in the new code by running pytest in the poetry-managed environment (in Python 3.11, but also CI checks this for 3.9, 3.10, and 3.11), and in the original codebase by installing its dependencies in a Python 3.10 virtual environment and running python main_text.py and python main_text.py --para.