step18: fix AttributeError: NoneType object has no attribute generation

[LeiLiGithub]

class Variable:
    # ...
    def backward(self, retain_grad=False):
        if self.grad is None:
            self.grad = np.ones_like(self.data)

        funcs = []
        seen_set = set()

        def add_func(f):
            if f not in seen_set:
                funcs.append(f)
                seen_set.add(f)
                funcs.sort(key=lambda x: x.generation)

        
        if self.creator is not None: # skip the first variable, whose creator is none
            add_func(self.creator)
    # ...