map_nested_tensor (#82240)
### Description
Example implementation of a slow utility function to make it easier to gain coverage.
### Issue
There's been various user requests for coverage (e.g. conv2d).
### Testing
This is a refactor. Existing tests apply.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82240
Approved by: https://github.com/jbschlosser
