Came here with the exact same problem. Funny though, I do recall the GPU being used when I first trained ML Image Classifier in CreateML.
dkga
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MacOS with AMD GPU here. I am using tensorflow for metal as soon as it was launched, with GPU acceleration. Sometimes I get the same message (Could not identify NUMA node of platform GPU ID 0, defaulting to 0. Your kernel may not have been built with NUMA support.). But, it still uses the GPU. You can check that by opening Activity Monitor, then pressing Cmd + 3 and Cmd + 4, which shows you GPU and CPU usage.
One potential problem for you, assuming it still is not benefitting from acceleration, could be that you are using Python 3.9. If I recall correctly, tensorflow for metal requires Python 3.8. That is what I am using, without any problems.
Hope the above can help you & others!