For the first time at least, I would follow how it is done in the tutorial (with ssd-mobilenet-v1), as I haven’t verified the other models with mb2 and VGG backbones are working later on in the pipeline (during the ONNX export and TensorRT import). That pytorch-ssd code was forked and I use it mainly for ssd-mobilenet-v1 because that seems to work well.
After you train your model, you can use the eval_ssd.py script from pytorch-ssd repo to test your output .pth checkpoint on a test image. This will confirm that the PyTorch model itself is good. Then you can convert it to ONNX.