[Nested Tensor] Add a NestedTensorUtils header and cpp file for organization (#84385)
# Summary
Trying to do some clean up into code structure for nested tensors. This introduces a utility header and cpp file that implements helper functions.
This is the initial PR in more clean up. The next would be separating out the all native functions that create nested tensors into their own file since they do not infact do math on nested tensors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84385
Approved by: https://github.com/mikaylagawarecki
