compute-summary and png-summary currently work with image classifiers
only.

When multi-processing, we want only one process to generate the
summary, while the other processes do nothing (lazy bums!)

Add an example of compressing OD pytorch models.

In this example we compress torchvision's object detection models - FasterRCNN / MaskRCNN / KeypointRCNN.
We've modified the reference code for object detection to allow easy compression scheduling with YAML configuration.