Add 'val' to  VALIDATION_KEYWORDS. 
This PR fixes #4839 by adding the word `"val"` to the `VALIDATION_KEYWORDS` so that the `load_dataset()` method with `imagefolder` (and probably, some other directives as well) reads folders named `"val"` as well.

I think the supported keywords have to be mentioned in the documentation as well, but I couldn't think of a proper place to add that.