All labs: Cache dataset locally

[willprice]

The CIFAR10 server is very unreliable and frequently returns 404 or 500 errors, we need to be able to run lab sessions when the server is down. Let's host the dataset locally on BC4 in some shared storage and use that.

[willprice]

We should try loading from a shared path on BC4 then only download locally if not present, lets stick it in the home directory so it can be reused across lab sessions