Refactor NT offsets metadata to be a Tensor (#96909)
It's tedious work, but somebody's gotta do it.
Benefits:
* Enable access to offsets metadata from Python via private API (for validation, etc.)
* Consistency with nested sizes / strides metadata
* Needed for SymInt-ifying offsets metadata
* more TBD
Bonus:
* Remove `_tensor` suffixes from metadata / getter names
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96909
Approved by: https://github.com/drisspg
