Remove manual download from imagenet-1k
_The documentation is not available anymore as the PR was closed or merged._ Thanks for the reviews @apsdehal and @lhoestq! As suggested by @lhoestq, I'll separate the train/val/test splits, apply the validation split fixes and shuffle the images files to simplify the script and make streaming faster. @apsdehal I dismissed your review as it's no longer relevant after the data files changes suggested by @lhoestq. 
Remove the manual download code from `imagenet-1k` to make it a regular dataset.