Document gotcha when using DataLoader with workers

[w-m]

Using .cache() (with the default memory cacher) does nothing when the Dataset is used in a multi-process DataLoader. This is a gotcha that should probably pointed out in the documentation and the tutorial, as it is easy to overlook.

In my case I was dropping in torchdatas cache in a program that already had the DataLoaders defined. The DataLoaders were initialized with a positive int in num_workers. It took me a while to figure out why the cache didn't seem to work, at all.