* Fix bitsandbytes zombie module breaking test collection
A partially failed `import bitsandbytes` leaves the package half-imported:
CPython evicts only the parent from sys.modules and keeps every submodule it
had already loaded. The next import re-executes __init__ but every
`from .x import y` is served from cache, so the submodule attributes are never
rebound. The package imports "successfully" while `bnb.functional` is gone.
Bind the submodule via `import bitsandbytes.functional as bnb_functional`,
which reads sys.modules directly and survives that state, and import
bitsandbytes in tests/conftest.py on the real CPU path before
torch.cuda.is_available() is mocked, so the half-imported state is never
created in the first place.
* [pre-commit.ci] auto fixes from pre-commit.com hooks
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