Allow ORT backend for DTensor (#101914)
fixes #101911
Currently, `DTensor` supports cuda and cpu. This PR makes some changes for easier integration with the ort backend.
* `Backend.NAME` attribute now has value `name` instead of `NAME` for backends registered through `register_backend(name)`; this matches the pattern for backends with built-in support like nccl.
* remove unused `_check_for_nccl_backend` function
* add test case that moves parameters to device in the `partition_fn` - a scenario that's useful for big models
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101914
Approved by: https://github.com/wanchaol
