Python code for making predictions with a traced torchscript Mask R-CNN model

[fracpete]

Similar to the torchscript_traced_mask_rcnn.cpp, the following Python code can be used for making predictions with a traced torchscript Mask R-CNN model:
https://github.com/fracpete/detectron2/blob/master/tools/deploy/torchscript_traced_mask_rcnn.py

I was missing an example on how to use a torchscript model from Python, so it took me a while figuring that out (see #2781). Other people might benefit from this example as well. Either grab the code directly or allow me to do a PR.