Allow data size equal to 0 for SegmentReduce (#99733)
Summary:
Support special case that data size can be 0 for SegmentReduce.
Example code below:
```
x = torch.ones((0, 6)).cuda()
lengths = torch.tensor([0, 0]).cuda()
torch.segment_reduce(x, "sum", lengths=lengths, unsafe=False, initial=0)
```
Previously, error message: Expected data.numel() > 0 to be true, but got false.
Now expect to return 0.
Test Plan: contbuild & OSS CI
Differential Revision: D45133827
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99733
Approved by: https://github.com/ngimel
