[ONNX] Fix sum export with attribute keepdims (#59316) (#60245)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60245
Fix after b9bdb07a0261ab5a0b1038f290fa03af6ce0415f. Improving previous fix on two aspects
* Not only checks 0 on first dimension for empty tensor.
* Do not assume empty tensor when shape is not accessible.
Test Plan: Imported from OSS
Reviewed By: zou3519, ZolotukhinM
Differential Revision: D29494917
Pulled By: SplitInfinity
fbshipit-source-id: 02587c3c3be0510312c1a1959f28cab12d81812d
Co-authored-by: BowenBao <bowbao@microsoft.com>
