Minimal NestedTensor (#72881)
Summary:
This PR adds a minimal version of a NestedTensor. It introduces the general harness future development can be built around.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72881
Reviewed By: albanD
Differential Revision: D34259177
Pulled By: cpuhrsch
fbshipit-source-id: 0245c36f603424e20f3b09651043c207f526d760
(cherry picked from commit 10764e8d427f29b364567e4cbc86ed73c3933158)
