Add NestedTensor specific dispatch key to PyTorch (#44668)
Summary:
This adds a dedicated dispatch key for the [nestedtensor project](https://github.com/pytorch/nestedtensor).
- [ ] Since this isn't a device or a backend, does this need further updates in other places other than DispatchKey.h?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44668
Reviewed By: zhangguanheng66, ailzhang
Differential Revision: D23998801
Pulled By: cpuhrsch
fbshipit-source-id: 133b5a9a04c4f61c27c0728832da09e4b38a5939
