[Caffe2][ThreadPool] Make sure numThreads does not exceed the number of big cores (#33523)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33523
When using `ThreadPool::setNumThreads` to set the number of threads, it should not exceed the number of big cores. Otherwise, the performance could degrade significantly.
Test Plan:
```
cd ~/fbsource/xplat
buck test caffe2:caffe2_testAndroid
```
Reviewed By: dreiss
Differential Revision: D19779267
fbshipit-source-id: 4e980e8a0ccc2f37e1c8ed16e2f4651d72924dbd
