New models and bug fixes

Improvements:

• Supports lazy loading for the Detectron2 module. Now the dependency for Detectron2 will be requested only when you explicitly create a Detectron2LayoutModel object. This might be helpful for using the plain layoutparser library without installing the Detectron2 module.

New models:

• Incorporated a pre-trained model based on the NewspaperNavigator dataset: lp://NewspaperNavigator/faster_rcnn_R_50_FPN_3x/config

Fixes:

• Corrected a bug in visualization that might overwrite original the image