[te] Use Dtype::is_signed instead of an ad hoc local predicate. (#49147)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49147
D25456366 adds Dtype::is_signed (which is backed by c10::isSignedType), so use that instead of this one-off.
ghstack-source-id: 118415315
Test Plan:
```
buck test //caffe2/test{:jit,:tensorexpr,/cpp/tensorexpr:tensorexpr}
```
Reviewed By: robieta
Differential Revision: D25456683
fbshipit-source-id: 428f1e8bff21ea05730690226a44984995c4c138
