Add SSD to benchmark model repo

[bertmaher]

This is in pytorch/hub: https://pytorch.org/hub/nvidia_deeplearningexamples_ssd/.

There's some evidence that this can be made much faster (up to 3x) with some graph optimizations: https://paulbridger.com/posts/video-analytics-deepstream-pipeline/

[desertfire]

https://pytorch.org/hub/nvidia_deeplearningexamples_ssd/ is highly coupled with Nvidia's DeepStream and TensorRT. I decide to pick a more general implementation from https://github.com/sgrvinod/a-PyTorch-Tutorial-to-Object-Detection.git .