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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def wd(self, val:float)->None:
        "Set weight decay."
        if not self.true_wd: self.set_val('weight_decay', listify(val, self._wd), bn_groups=self.bn_wd)
        self._wd = listify(val, self._wd)