Fix unused imports in torch/onnx/symbolic_opset8.py (#23678)
Summary:
Which causes lint errors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23678
Differential Revision: D16622458
Pulled By: mrshenli
fbshipit-source-id: 145ad30dfb452dd556573c1b3d4cdd9cd7852752
