Move code to code block in Youtube "Training with PyTorch" module

[kiersten-stokes]

This PR corrects the formatting of a non-executable block of code in this tutorial.

Previously, the code is rendered correctly in the html, but shows up as follows in the notebook file (screenshot from Colab):

With these changes the same block is displayed correctly. The block should remain non-executable, as it is given as an example only and PATH is not defined earlier.

Alternatively, I could change the PATH variable to model_path and change the block into an executable one, but this concept is covered in other tutorials so this may be unnecessary.

cc @suraj813

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