They write that they use PrivateUse1, so it’s a custom out-of-tree backend

I attended the related session at Next’26 yesterday. From my understanding it is a new backend and they will release the torch tpu source on github in one or two months. It will not support all ops initially but they are moving fast. Still for a while torchax is mature enough to run torch models on tpus by translating to jax.