`NUM_THREADS` leave at least 1 CPU free (#5706)

Updated strategy leaves at least 1 cpu free to avoid system overloads. Partially addresses https://github.com/ultralytics/yolov5/issues/5685

utils/datasets.py

@@ -35,7 +35,7 @@ HELP_URL = 'https://github.com/ultralytics/yolov5/wiki/Train-Custom-Data'
 IMG_FORMATS = ['bmp', 'jpg', 'jpeg', 'png', 'tif', 'tiff', 'dng', 'webp', 'mpo']  # acceptable image suffixes
 VID_FORMATS = ['mov', 'avi', 'mp4', 'mpg', 'mpeg', 'm4v', 'wmv', 'mkv']  # acceptable video suffixes
 WORLD_SIZE = int(os.getenv('WORLD_SIZE', 1))  # DPP
-NUM_THREADS = min(8, os.cpu_count())  # number of multiprocessing threads
+NUM_THREADS = min(8, max(1, os.cpu_count() - 1))  # number of multiprocessing threads
 
 # Get orientation exif tag
 for orientation in ExifTags.TAGS.keys():