[aten] fix shadowing variable warning (#32573)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32573
Fix the following warning
```
caffe2/aten/src/ATen/ParallelOpenMP.h:36:9: warning: declaration of ‘num_threads’ shadows a previous local [-Wshadow=compatible-local]
int64_t num_threads = omp_get_num_threads();
^~~~~~~~~~~
caffe2/aten/src/ATen/ParallelOpenMP.h:29:9: note: shadowed declaration is here
int64_t num_threads = omp_in_parallel() ? 1 : omp_get_max_threads();
^~~~~~~~~~~
```
Test Plan: CI
Reviewed By: ilia-cher
Differential Revision: D19552578
fbshipit-source-id: b8388de1aaa2bb7676b777c93b8ba9c25f5a3d51
