Add warning information for dtypetensor. (#99521)
Fixes #ISSUE_NUMBER
Without affecting the existing cpu/cuda logic, a separate interface is provided for the custom backend and users can choose whether to use the interface function which provides 10 tensor types with custom backend variations.
Therefore, users can use torch.set_deafult_tensor_type to set the default device tensor type, or use torch.xxx.dtypetensor to create a tensor.For example，torch.set_deafult_tensor_type(torch.foo.DoubleTensor) or torch.foo.DoubleTensor([]).
@albanD , please review my changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99521
Approved by: https://github.com/albanD
