I believe huiquing might be right.
In the network overview image, there appears to be a mismatch between the displayed output dimensions of the features (at each step, up to the fully connected layer) and the actual feature dimensions you get with the code presented in the tutorial (I haven’t checked the colab).
For example, the C1 feature maps dimensions should be 6@30*30 (and not 6@28*28, as in the image) for an input image of 32*32 and Conv2d layer with 6 output channels, a 3*3 kernel (and padding=0 and stride of (1,1), as per default parameter values). The same applies for the subsequent features  (at each step, up to the fully connected layer). The code ends up working, but this might be misleading for someone reading the tutorial.
It seems that tutorial is referencing Pytorch version 1.4, though, so maybe it has already been corrected in version 1.5?