Add nested tensor metadata nested_stride then use it in unbind, select (#79831)
2 reasons to add metadata `nested_stride`:
1. it will be used later in `reshape` and `transpose`
2. it reduces the computation to get offsets and shapes necessary in `unbind`-like codes, which will be used again and again in nested tensor operations
`unbind` and `select` are refactored to make use of `nested_stride`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79831
Approved by: https://github.com/cpuhrsch, https://github.com/jbschlosser
