Add mechanism to pass a number of workers to cpp extensions (#33346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33346
Fixes #33091
This PR lets users control the number of workers that cpp extensions
uses through the environment variable `MAX_JOBS`. If the environment
variable is a non-negative integer we use that many threads; otherwise,
ninja falls back to the default.
I chose to use the name `MAX_JOBS` because we use it in PyTorch already
to control the number of workers PyTorch builds with. There is a risk
that users of cpp extensions already have `MAX_JOBS` set but we are
hoping that that risk is small and/or it means semantically the same
thing.
Test Plan: - tested locally
Differential Revision: D19911645
Pulled By: zou3519
fbshipit-source-id: d20ed42de4f845499ed38f1a1c73e9ccb620f780
