Method forward

code/chapter9.py:1130–1136

(self, X_train)

        self.is_initialized = True
    
    def forward(self, X_train):
        Xs = {}
        out = X_train
        for k, v in self.layers.items():
            Xs[k] = out
            out = v.forward(out)
        return out, Xs
    
    def backward(self, grad):
        dXs = {}