Make codeblocks non-executable

[jlin27]

Certain codeblocks in the Training a Classifier colab should be non-executable; otherwise, users will run and expect output and be confused when they get an error. Specifically the code under the "2. Define a Convolution Neural Network" and the "3. Define a Loss function and optimizer" sections.

They should match the html version: https://pytorch.org/tutorials/beginner/blitz/cifar10_tutorial.html#sphx-glr-beginner-blitz-cifar10-tutorial-py