Add custom backend case for storage and automatically generate storage attributes. (#98478)
Currently storage only considers partial backend. We want storage to create on custom backend by key PrivateUse1.
It also provides an easy automatic generation of storage-related attributes.
When the user registers a new backend, the corresponding methods and attributes can be automatically generated.
Do this code.
`torch.utils.rename_privateuse1_backend('foo')`
`torch.utils.generate_storage_for_privateuse1_backend()`
Then, get the following methods and attributes.
`torch.TypedStorage.is_foo`
`torch.TypedStorage.foo()`
`torch.UntypedStorage.is_foo`
`torch.UntypedStorage.foo()`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98478
Approved by: https://github.com/albanD
