Use unbind for tensor.__iter__ (#40884)
Summary:
Unbind, which has a special backward with cat, is arguably better than multiple selects, whose backward is creating & adding a bunch of tensors as big as `self`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40884
Reviewed By: pbelevich
Differential Revision: D22363376
Pulled By: zou3519
fbshipit-source-id: 0911cdbb36f9a35d1b95f315d0a2f412424e056d
