Added a filesize check to local dataset cache

I ran into the problem where I was experimenting with my own dataset and local caching. When I updated the dataset, the `LocalDatasetCache` just compared filenames and used the outdated locally stored version. This can be pretty tricky to debug, because the model trains just fine, but it's not training on the data that you've given.

I'm just comparing file sizes here, so it's not foolproof, but it doesn't require the file to be hashed or compared byte-wise.