cpu support

[Ricardicus]

First of all, tank you Andrej Karpathy for the amazing Youtube series you have done on deep learning and for being such a great educator on the subject online.

Regarding this PR. I noticed that there was support for the poor mans configurator as well as data loader but I failed to find support for the poor mans hardware. I therefore added it. It might also be useful in case high performance, or performance in general, isn't a requirement or one would like to use the computer as some sort of electric heating system.

# in case there is no GPU but a lot of time to spend
python train.py finetune_shakespeare --device=cpu --init_from=gpt2