[ONNX] Support nonzero(*, as_tuple=True) export (#47421)
Summary:
Support exporting with `as_tuple = true`
Example:
`torch.nonzero(x, as_tuple=True)`
This is the same as
`torch.unbind(torch.nonzero(x), 1)`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47421
Reviewed By: malfet
Differential Revision: D24870760
Pulled By: bzinodev
fbshipit-source-id: 06ca1e7ecf95fbf7c28eebce800df958c83264c8
