Add ImageFolderWithoutTargets

[desimone]

Useful data loading in instances where you need data from disk, but don't have labels, or the labels aren't in the form of {root}/{category}/{image}.

Don't love the class name.

[apaszke]

I think we should revert the class naming i.e. have ImageFolder be something that just globs the images and ImageClassFolder (or similar) to load the currently supported structure.

[desimone]

I think that makes sense. It would be cool if we could also have a flag on ImageFolder to allow recursive directory walking too.

I'm happy to make those changes but know it would break some existing code in examples and in other places as well.

[apaszke]

Yeah, we wanted to make that change before the release but we didn't make it. If you make this change I can go over examples. We'll put that as an API change for the next release.

[soumith]

hmmm, not a good idea to change the name now. ImageFolder is being used everywhere (many projects outside of examples). Best we can do is give a keyword argument to ImageFolder that add the behavior in this PR

[yanghuanflc]

I maintained my own version with a dataset class named as ImageReference, which using a reference txt file to indicate the image paths and labels, then you can load images from anywhere you want only with a reference file. I can setup a PR with it if needed.