# Context
In pytorch#161183, we added NUMA-binding support for `Callable` entrypoints to `elastic_launch`.

However, we would raise an exception if the subprocesses would be spawned in parallel via `ThreadPoolExecutor`, which is an option configurable via the `TORCH_MP_PARALLEL_START` environment variable (see diff).

The logic here was that `os.sched_setaffinity`, which we used to set CPU affinities, is [per process](https://docs.python.org/3/library/os.html#os.sched_setaffinity), so there could be a race condition during a parallel start:

> Restrict the process with PID pid (or the current process if zero) to a set of CPUs. mask is an iterable of integers representing the set of CPUs to which the process should be restricted.

But on further reading, the Linux docs say [`sched_setaffinity` is per *thread*.](https://man7.org/linux/man-pages/man2/sched_setaffinity.2.html) As it turns out, the Python doc is a misnomer.

I [verified that `sched_setaffinity` only affects the calling thread, not the entire calling process.](https://gist.github.com/pdesupinski/7e2de3cbe5bb48d489f257b83ccddf07)

The upshot is that we actually *can* safely use the inheritance trick from pytorch#161183 even with parallel start, since the setting will be inherited from the calling thread, and `os.sched_setaffinity` only affects the calling thread.

# This PR
Remove restrictions against parallel start for NUMA binding.

Pull Request resolved: pytorch#161576
Approved by: https://github.com/d4l3k