register vs rename privateuse1_backend

[artyom-beilis]

I notices in the code you use register_privateuse1_backend('foo') while in nighly it didn't work but torch.utils.rename_privateuse1_backend('foo') this works.

Is it going to be the future API?

I've updated my OpenCL backend https://github.com/artyom-beilis/pytorch_dlprim/tree/true_out_of_tree_support to use it with nighly version of pytorch and ohhh it so simplified backend instalation! Just few commands and it is good to go!