Add support for .to() for NestedTensor backends (#87146)
Summary: This commit adds support for moving NestedTensors from CPU to GPU and back. The implementation includes requires implementing empty_like(), which is based on PR#83140.
Test Plan: Added a new unit test based on the unit test for the main .to() implementation. All unit tests must pass, as well as every sandcastle job.
Differential Revision: D40437585
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87146
Approved by: https://github.com/drisspg
Author
Antoni Viros i Martin
