add an API for external backends to register custom device names (#86992)
This API adds some improvements to external backends who are building C++ backends out of tree using the `PrivateUse1` dispatch key.
The docs and linked examples go over the API in more detail, but you should be able to use it like:
```
# This should probably be in the __init__.py file of a external backend's python package
> torch.register_privateuse1_backend("foo")`
# And it will allow the user to do this:
> a = torch.ones(2, device="foo")
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86992
Approved by: https://github.com/albanD
