Allow set pred with ConstantVariable (#98900)
It's part of the effort to improve PT2 Export UX. This PR is to improve the usability of `torch.cond()` by allowing user to set `pred` as `ConstantVariable` as it's not often to see control flow on rank or a tensor or dim size which is traced as `ConstantVariable`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98900
Approved by: https://github.com/jansel
