Fix Caltech256 links

[applecuckoo]

Direct mentioning @alex000kim for review.

This PR fixes any references of Caltech256 to point towards a more current link at data.caltech.org since the previous link is dead.

I also took a look at the Danbooru series of datasets and it seems like they have moved it to a different rsync server. From what I found, it seems that the Danbooru2018 dataset that was previously used can be subsetted out of the current dataset by ignoring all images after ID #3,368,713. I haven't made any changes to 3_optional_download_drawings_.sh since I'm not one to decide whether to switch to Danbooru2021.

[moluzhang]

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[longems]

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