[BE] Improve has_bf16_support (#57408)
Summary:
Use `functools.lru_cache` to avoid calling this function multiple time
Check that we are running on Linux platform before trying to open
"/proc/cpuinfo"
Do not spawn new process, but simply open("/proc/cpuinfo").read() and
search the output for the keywords
Fixes https://github.com/pytorch/pytorch/issues/57360
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57408
Reviewed By: driazati
Differential Revision: D28136769
Pulled By: malfet
fbshipit-source-id: ab476774c3be2913cb576d98d47a2f7ec03c19aa
