Scale SGDW parameters by (1 - lr * weight_decay)

[shaneraphel]

Summary

Fixes #454 and the same mismatch reported in #433. Decoupled weight decay, as written by Loshchilov and Hutter (arXiv 1711.05101), scales the parameter and then subtracts the step:

θ ← (1 − λ μ) θ
θ ← θ − μ m

SGDW.step did the gradient step and then called p.add_(weight_decay, alpha=-lr), which subtracts the constant λμ from every coordinate. On a parameter of 4, with learning rate 0.1, weight decay 0.2 and a zero gradient, that lands on 3.98. The paper scale lands on 3.92. With a gradient of 1.5 the paper order (scale, then subtract) lands on 3.77. The same library's Lion, QHAdam, AdaBelief and Sophia already scale by (1 - lr * weight_decay). SGDW was the one that added a constant, which is also why it trained worse than SGD at the same hyperparameters.

The decay now happens before the step, and only when weight decay is non-zero. A zero weight decay still subtracts lr times the gradient, including the first momentum step.

Test plan

• pytest tests/test_sgdw.py — zero gradient scales [4, -2] to [3.92, -1.96]; gradient 1.5 on 4 lands on 3.77; weight decay 0 with momentum 0.9 matches a plain SGD step.

This change was produced with assistance from an AI coding tool. I compared the update with the paper formula, changed the decay to a scale, and ran the tests locally.