Update torch::lazy::BackendDevice to have a new default ordinal (#76264)
Summary:
Fixes https://github.com/pytorch/xla/issues/3490. Updates `torch::lazy::BackendDevice` with changes below:
1. Remove the no-op string constructor.
2. Update default ordinal to `-1`.
3. Add a `is_valid` function to check if `ordinal` is valid/non-default (`ordinal >= 0`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76264
Reviewed By: mrshenli
Differential Revision: D35860266
Pulled By: alanwaketan
fbshipit-source-id: 554ebe16a0683d37b00270c4f35163bf690bfe28
(cherry picked from commit b941d10e8545dfecfb34e4d5c24a29a1cc49bc4b)
