4_cifar10_tutorial.ipynb should be added plt.show()

[btdan]

https://github.com/zergtant/pytorch-handbook/blob/master/chapter1/4_cifar10_tutorial.ipynb in this webpage, the function imshow(img) should be modified. It should be added a new line at the end: plt.show() or else, there will be no images shown at all. Thanks for you.