Fix OMP_NUM_THREADS=1 for macOS (ultralytics#8624)

Resolves ultralytics#8623

Author: glenn-jocher

utils/general.py

@@ -52,7 +52,7 @@
 pd.options.display.max_columns = 10
 cv2.setNumThreads(0)  # prevent OpenCV from multithreading (incompatible with PyTorch DataLoader)
 os.environ['NUMEXPR_MAX_THREADS'] = str(NUM_THREADS)  # NumExpr max threads
-os.environ['OMP_NUM_THREADS'] = str(NUM_THREADS)  # OpenMP max threads (PyTorch and SciPy)
+os.environ['OMP_NUM_THREADS'] = '1' if platform.system() == 'darwin' else str(NUM_THREADS)  # OpenMP (PyTorch and SciPy)
 
 
 def is_kaggle():