Multiprocessing option for generator

[ap-buf]

I was at one point using the Simple Generic Generator similar to this example: https://github.com/Kitware/nrtk/blob/main/examples/simple_generic_generator.ipynb
to generate some metrics

However, there was a main issue that led me away

1. Because the generator obscures the predict function inside of the detector, I could not use multiprocessing and for use in a web app, the speed is not sufficient without multiprocessing. Perhaps a multi-processing option could be added to the generator.