Deprecation warning in Tensor.storage() should suggest alternate API

[kurtamohler]

Since #85303 deprecated TypedStorage, calling Tensor.storage() now raises a warning about the deprecation because it returns a TypedStorage. The warning should be updated to mention how to get an UntypedStorage from the tensor. At the moment, this is done with Tensor._storage(), but it should be renamed to Tensor.untyped_storage() so that it's public, and there should be documentation for it.

It might be good for the warning to mention in which cases UntypedStorage and TypedStorage are not interchangeable.

We should also have a function that takes an UntypedStorage and a torch.dtype and returns a tensor that wraps the storage. EDIT: torch.tensor does this

cc @albanD

[albanD]

That sounds good. The new untyped_storage API sounds good.
Not sure if we really need an API to build a Storage again? The APIs that we have today that take Storage also accept UntypedStorages right?

It would be good as well to provide 1-1 correspondance for python storage object with c++ storage like we do for Tensor. @ezyang do you think it is worth reproducing all the complexity we have for Tensor here?

[ezyang]

@eellison has been asking for it and I think is reasonable