Use cpuinfo to determine c10::ThreadPool thread number (#107010)
This PR prefers "logical processor number" (the cpu cores shown in htop) returned by cpuinfo for determining c10 thread number. If that fails, it uses hardware_concurrency exactly.
The motivation is that in a x86 host with 64 cores and Hyper-Threading disabled, the current behavior uses 32 threads, resulting half of cores being idle.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107010
Approved by: https://github.com/ezyang
