The code behind these timings can be found at https://github.com/stsievert/pytorch-timing-comparisons 112 in Jupyter notebooks. They are timing a CPU tensor to NumPy array, for both tensor flow and PyTorch.
I would expect that converting from a PyTorch GPU tensor to a ndarray is O(n) since it has to transfer all n floats from GPU memory to CPU memory. I’m not sure on the O constant, but I would expect it to be fairly small.