If you really want to do the simplest, I would suggest:
class Autoencoder(nn.Module):
    def __init__(self, ):
        super(Autoencoder, self).__init__()
        self.fc1 = nn.Linear(784, 32)
        self.fc2 = nn.Linear(32, 784)
        self.sigmoid = nn.Sigmoid()

    def forward(self, x):
        x = self.sigmoid(self.fc1(x))
        x = self.sigmoid(self.fc2(x))
        return x