`voc_classes()` need refactor

[cregouby]

Current situation

voc_classes() are currently defined in R/models-deeplabv3.R
pascal_voc_datasets() as well as pascal_segmentation_dataset() and pascal_detection_dataset() all expose a $classes attribute that is not aligned with thoses classes

• in the code
• in the tests
• in the examples

Expected situation

a renamed of aliases for pascal_voc_classes() to be consistent with dataset names
relocate the voc_classes() into R/dataset-pascal.R
a consistent, tested and documented use of pascal_voc_classes() for all pascal datasets

[sau1812]

Hi @cregouby! I'd love to help with this refactor as my first contribution. Could you please assign this to me?