Misc improvements

[lolipopshock]

1. When loading Layout Parser official models, Detectron2LayoutModel can automatically detect the label_map, . For example,

   model = lp.Detectron2LayoutModel("lp://HJDataset/faster_rcnn_R_50_FPN_3x/config")
   model.label_map
   # {1: 'Page Frame', ... }

2. Detectron2LayoutModel now supports the enforce_cpu flag that enforces using cpu even when CUDA devices are available. Inspired by Change for personnal work #16 .

3. For visualization.daw_box, it now supports a show_element_type flag that shows the bbox category name on the top left corner of the layout objects.