Update nonzero onnx export (#18047)

Update nonzero onnx export (#18047)

Summary:
The output format of NonZero in ONNX(numpy https://docs.scipy.org/doc/numpy/reference/generated/numpy.nonzero.html) differs from that in PyTorch:
In ONNX: `[rank_of_input, num_of_nonzeros]`, whereas in PyTorch: `[num_of_nonzeros, rank_of_input]`.
To resolve the difference a Transpose op after the nonzero output is added in the exporter.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18047

Differential Revision: D14475081

Pulled By: ezyang

fbshipit-source-id: 7a3e4899f3419766b6145d3e9261e92859e81dc4