keras/src/regularizers/regularizers.py/L1L2/__init__
class L1L2:    def __init__(self, l1=0.0, l2=0.0):
        # The default value for l1 and l2 are different from the value in l1_l2
        # for backward compatibility reason. Eg, L1L2(l2=0.1) will only have l2
        # and no l1 penalty.
        l1 = 0.0 if l1 is None else l1
        l2 = 0.0 if l2 is None else l2
        validate_float_arg(l1, name="l1")
        validate_float_arg(l2, name="l2")

        self.l1 = l1
        self.l2 = l2