Rename DispatchKey.PrivateUse1 to custom device in torchgen. (#99406)
I want to use torchgen to generate code, and my yaml file format is the same as `native_functions.yaml`.
I will use the PrivateUse1, but in my yaml file, I don't want to show PrivateUse1 to the user.
So I want to achieve the following result(e.g. my device is `YPU`):
```
>>>from torchgen.model import DispatchKey
>>>str(DispatchKey.PrivateUse1)
"YPU"
>>>DispatchKey.parse("YPU")
DispatchKey.PrivateUse1
```
I also thought that not everyone would need this feature, so I add a new func to handle this scenario.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99406
Approved by: https://github.com/ezyang
