Updates nonzero's as_tuple behavior to no longer warn. (#45413)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/44284.
[torch.nonzero](https://pytorch.org/docs/master/generated/torch.nonzero.html?highlight=nonzero#torch.nonzero) is distinct from [numpy.nonzero](https://numpy.org/doc/1.18/reference/generated/numpy.nonzero.html?highlight=nonzero#numpy.nonzero). The latter returns a tensor by default, and the former returns a tuple of tensors. The `as_tuple` argument was added as part of an intended deprecation process to make torch.nonzero consistent with numpy.nonzero, but this was a confusing change for users. A better deprecation path would be to offer torch.argwhere consistent with [numpy.argwhere](https://numpy.org/doc/stable/reference/generated/numpy.argwhere.html?highlight=argwhere#numpy.argwhere), which is equivalent to the default torch.nonzero behavior. Once this is offered a change to torch.nonzero should be more straightforward with less user disruption, if we decided that's the correct change to pursue.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45413
Reviewed By: ngimel
Differential Revision: D23975015
Pulled By: mruberry
fbshipit-source-id: b59237d0d8c2df984e952b62d0a7c247b49d84dc
