[inductor] subproc parallel-compile: start thread last in init (pytor…

…ch#128037)

Summary: Observed on an internal workload: the helper thread started and attempted to access member variables before they were initialized.

Differential Revision: [D58239827](https://our.internmc.facebook.com/intern/diff/D58239827)
Pull Request resolved: pytorch#128037
Approved by: https://github.com/Skylion007, https://github.com/eellison

torch/_inductor/compile_worker/subproc_pool.py

@@ -90,14 +90,17 @@ def __init__(self, nprocs: int):
         self.write_lock = threading.Lock()
         self.read_pipe: Pipe = typing.cast(Pipe, self.process.stdout)
         self.read_thread = threading.Thread(target=self._read_thread, daemon=True)
-        self.read_thread.start()
 
         self.futures_lock = threading.Lock()
         self.pending_futures: Dict[int, Future[Any]] = {}
         self.job_id_count = itertools.count()
 
         self.running = True
 
+        # Start thread last to ensure all member variables are initialized
+        # before any access.
+        self.read_thread.start()
+
     def submit(self, job_fn: Callable[..., Any], *args):
         if args:
             job_fn = functools.partial(job_fn, *args)