improve readability of the cifar10 blitz tutorial code

[wAuner]

When I was working through the blitz cifar 10 tutorial it took me quite some time to understand the evaluation logic of the per-class accuracy.
I've added some code comments and rewrote this small part to make it much more obvious what is going on. Generally I've tried to replace the ranges with fixed numbers for batchsize or classes with speaking names.
In detail:

• now using dicts with classnames as keys to collect classifications, instead of lists which map to class by index
• implicitly removed an unneccessary squeeze operation diffline271
• removed the unexplained reference to the batchsize (hardcoded int)
• make the counting of the correct predictions per class more readable

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Deploy preview for pytorch-tutorials-preview ready!

Built with commit ca8fb70

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[holly1238]

Hi @wAuner! Can you sign the CLA agreement so that I can merge the pull request? Thanks!

[wAuner]

@holly1238 if it's not some 100 page document written in legal Klingon, then yes 😄

[wAuner]

@holly1238 ok I've signed that cla. Maybe the check needs to be triggered again

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