Handle interval correctly for categories

If the transformer does not handle interval and the original dimension
is a category, then we should return the categories. We cannot transform
them.

src/orion/core/worker/transformer.py

@@ -436,7 +436,7 @@ def interval(self, alpha=1.0):
             interval = self.transformer.interval()
             if interval:
                 return interval
-        elif self.original_dimension.prior_name == 'choices':
+        if self.original_dimension.type == 'categorical':
             return self.original_dimension.categories
 
         low, high = self.original_dimension.interval(alpha)