Dear SherlockLiao and hughperkins, thank you very much! I would like work it out.
Indeed, it worked according the suggestion of SherlockLiao though the net is not stable. (I work out two times for 8 times.)
# Define model: 
class net(nn.Module):
    def __init__(self):
        super(net, self).__init__()
        self.fc1 = nn.Linear(W_target.size(0), 1)
        self.fc2 = nn.Linear(2, 1)

    def forward(self, x):
        x1 = self.fc1(x)
        y = torch.cat([x1 ** i for i in range(1, 3)], 1)
        # print(y)
        y_pred = self.fc2(y)
       return y_pred

net1 = net()