Fix AttributeError in auto-augment factories when hparams is None - #2747

What

The three public auto-augment factory functions crash on their own documented default hparams=None:

from timm.data.auto_augment import (
    auto_augment_transform, rand_augment_transform, augment_and_mix_transform,
)

augment_and_mix_transform('augmix-m5-w4-d2')   # AttributeError: 'NoneType' object has no attribute 'setdefault'
rand_augment_transform('rand-m9-n3-mstd0.5')   # AttributeError
auto_augment_transform('original-mstd0.5')     # AttributeError

Each of those config strings is copied verbatim from the corresponding function's own docstring example.

Why

All three functions declare hparams: Optional[Dict] = None, but parse the mstd/mmax config sections by calling hparams.setdefault(...) before hparams is normalized:

• auto_augment_transform — hparams.setdefault('magnitude_std', ...) (mstd path)
• rand_augment_transform — hparams.setdefault('magnitude_std'/'magnitude_max', ...) (mstd/mmax paths)
• augment_and_mix_transform — same for mstd, plus an unconditional hparams.setdefault('magnitude_std', float('inf')), so it raises on any config string, not just ones containing mstd.

The ops helpers these factories call (auto_augment_policy, rand_augment_ops, augmix_ops, AugmentOp) already guard with hparams = hparams or _HPARAMS_DEFAULT, but the factories dereference hparams before delegating, so that guard is never reached. The internal transforms_factory.py path always passes a real dict, which is why this wasn't caught in normal training runs — it only bites callers who use the factories directly with the documented default.

Fix

Normalize hparams at the top of each factory, mirroring the existing hparams = hparams or _HPARAMS_DEFAULT idiom — but using a copy (dict(_HPARAMS_DEFAULT)). The factories mutate hparams via setdefault, and _HPARAMS_DEFAULT has no magnitude_std key, so reusing the shared module-level dict directly would leak magnitude_std into it and pin that value for every later call that relies on the defaults. Passing a real hparams dict is unchanged.

Tests

Added a parametrized regression test in tests/test_data.py covering every factory and each affected path (the mstd/mmax sections and augmix's unconditional default), asserting the transforms build and that the shared _HPARAMS_DEFAULT is never mutated. It fails on main (6 cases) and passes with the fix; tests/test_data.py + tests/test_factory.py stay green (52 passed).