Fix a shadowed variable (#64695)

Fix a shadowed variable (#64695)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64695

Resolves this warning:
```
caffe2/aten/src/ATen/ParallelOpenMP.h:109:63: warning: declaration of 'int64_t begin' shadows a parameter [-Wshadow=compatible-local]
  109 |   internal::invoke_parallel(begin, end, grain_size, [&](int64_t begin, int64_t end) {
      |                                                       ~~~~~~~~^~~~~
caffe2/aten/src/ATen/ParallelOpenMP.h:86:1: note: shadowed declaration is here
   85 | inline scalar_t parallel_reduce(
      |                 ~~~~~~~~~~~~~~~~
   86 |     const int64_t begin,
      | ^   ~
```

Test Plan: Sandcastle

Reviewed By: ngimel

Differential Revision: D30816128

fbshipit-source-id: 3adff6d94eea9fbd65885e88283cae10b87dba18