“Are you seeing the same error without using QAT?”
Yes.
I am seeing this link indicating that Can’t save a model with torch.save if model has a torch.Device attr #7545  and I tried to remove the self.device attribute in SSD class and still not working.
I did try to run the code you suggested in the link to exam where is the error by pickle_trick() it:

net = create_squeezenet_ssd_lite(no_classes, is_test=True)
print(pf(pickle_trick(net)))

But the code crashes indicating there is exception caught within the exception even I increase the maximum depth to 10,000.