Add a NestedTensor Readme (#91472)
# Summary
This PR adds a NestedTensor Readme which explains the code structure and will hopefully serve as a reference point for new contributors, especially if they would like to implement a NestedTensor kernel implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91472
Approved by: https://github.com/mikaylagawarecki, https://github.com/cpuhrsch
