[Nested Tensor]Clean up offsets (#84145)
# Summary
- Document contiguous offset construction
- Expand offsets by 1 so that storage offsets for `ntensor[i] = offsets[i+1] - offsets[i]`
Another simple one. While looking into this issue https://github.com/pytorch/pytorch/issues/84082 I noticed that the kernels essentially rebuild the offsets but with the added last element. I added this and also cleaned up the code a little
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84145
Approved by: https://github.com/albanD
