Add ONNX export support for multidim torch.sum. (#22240)
Summary:
This change fixes the issue reported in https://github.com/pytorch/pytorch/issues/22066.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22240
Reviewed By: zrphercule
Differential Revision: D15996934
Pulled By: houseroad
fbshipit-source-id: 3a842ba26f54aa710233fbe87d727fc1f2568d9c
