[PyTorch] Call self._impl.unbind directly in _nestedtensor wrapper (#74001)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74001
Incremental step toward removing this wrapper.
ghstack-source-id: 151540965
Test Plan: existing test_nestedtensor
Reviewed By: ezyang
Differential Revision: D34689382
fbshipit-source-id: 46eb203d4fc9ccc30008c7dc3ada24cab47b57d4
(cherry picked from commit 30e204362ca6a3da05e6d99726489879c9253a0e)
