torch.utils.rename_privateuse1_backend — PyTorch 2.4 documentation (and the pages linked from there) give some details but a lot is missing indeed.
I am hopping that this stack of PRs Add device daemon by albanD · Pull Request #131814 · pytorch/pytorch · GitHub , by providing a fully working example, will provide some much needed insights into what is happening.
It is still WIP progress though (see the readme there for the state and next steps).