Some things: A "single TPU" is 4 ASICs. It is not clear if it makes sense to compare a "single TPU" to a "single GPU." As a point of reference, NVIDIA's numbers are 6 hours for Resnet-50 on Imagenet when training with 8xV100. From a naive extrapolation, 4xV100 would probably take ~12 hours and 1xV100 about two days. Google has previously only compared TPUs to K80, so it will be interesting to see some benchmarks that…
Why does the number of chips matter?
Put another way, suppose Google tomorrow announced Cloud TPU v3 which was one ASIC identical in all ways to four v2 ASICs glued together. Would that be notable in any way? Seems like it would be a nop to me.
I think what matters is, how fast can you train a model, and at what cost? Doesn't really matter if it's one chip or 10,000 behind the scenes.