keras/src/backend/torch/optimizers/torch_optimizer.py/TorchOptimizer/_apply_weight_decay
class TorchOptimizer:    def _apply_weight_decay(self, variables):
        if self.weight_decay is None:
            return

        torch._foreach_mul_(
            [v.value for v in variables if self._use_weight_decay(v)],
            1 - self.weight_decay * self._get_current_learning_rate(),
        )