Add ImageNet-Sketch dataset
_The documentation is not available anymore as the PR was closed or merged._ I think you can go ahead with uploading the data, and also ping the author in parallel. I think the images may subject to copyright anyway (scrapped from google image) so the dataset author is not allowed to set a license to the data.

I think it's fine to upload the dataset as soon as we mention explicitly that the images may be subject to copyright.
This PR adds the ImageNet-Sketch dataset and resolves #3953 .