Minor fix for onnx ConstantOfShape export (#18199)
Summary:
Set value as tensor of 1 element instead of scalar, according to ONNX spec.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18199
Reviewed By: dzhulgakov
Differential Revision: D14542588
Pulled By: houseroad
fbshipit-source-id: 70dc978d870ebe6ef37c519ba4a20061c3f07372
