[JIT] Fix torch.jit.is_tracing() (#42486)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42486
**Summary**
This commit fixes a small bug in which `torch.jit.is_tracing()` returns
`torch._C.is_tracing`, the function object, instead of calling the
function and returning the result.
**Test Plan**
Continuous integration?
**Fixes**
This commit fixes #42448.
Test Plan: Imported from OSS
Reviewed By: bertmaher
Differential Revision: D22911062
Pulled By: SplitInfinity
fbshipit-source-id: b94eca0c1c65ca6f22acc6c5542af397f2dc37f0
