Fix bug in `ImageClassifcation` task template
The CI failures are unrelated to the changes in this PR. > The CI failures are unrelated to the changes in this PR.

It seems that some of the failures are due to the tests on the dataset cards (e.g. CIFAR, MNIST, FASHION_MNIST). Perhaps it's worth addressing those in this PR to avoid confusing downstream developers who branch off `master` and suddenly have a failing CI? @lewtun We only run these tests against the modified datasets on the PR branch, so this will not lead to errors after merging.
Fixes a bug in the `ImageClassification` task template which requires specifying class labels twice in dataset scripts. Additionally, this PR refactors the API around the classification task templates for cleaner `labels` handling.

CC: @lewtun @nateraw