Implement named inference rule for torch.sum
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/23081
Test Plan:
- New tests [namedtensor ci]
Imported from OSS
Differential Revision: D16419174
Pulled By: zou3519
fbshipit-source-id: 8679f77f121664d0398d7f062a53c0fa37482481
