Rename `classes` to `num_classes` throughout KerasCV

[ianstenbit]

For consistency with the KerasNLP API, let's always refer to the number of classes in a classifier, OD model, etc., as num_classes. (Currently, classes is used in many places)

[ID6109]

Can I take up this issue?

[ianstenbit]

@ID6109 I'm actually about to open a PR for this one

I think it's going to cause a lot of merge conflicts with some other open PRs so I want to be able to control it so we don't disturb other contributors.

[ID6109]

Sure @ianstenbit , no worries.