Add `--workers 8` argument to val.py (#5857)

* Update val.py

Add an option to choose number of workers if not called by train.py

* Update comment

* 120 char line width

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

val.py

         iou_thres=0.6,  # NMS IoU threshold
         task='val',  # train, val, test, speed or study
         device='',  # cuda device, i.e. 0 or 0,1,2,3 or cpu
+        workers=8,  # max dataloader workers (per RANK in DDP mode)
         single_cls=False,  # treat as single-class dataset
         augment=False,  # augmented inference
         verbose=False,  # verbose output
         pad = 0.0 if task == 'speed' else 0.5
         task = task if task in ('train', 'val', 'test') else 'val'  # path to train/val/test images
         dataloader = create_dataloader(data[task], imgsz, batch_size, stride, single_cls, pad=pad, rect=pt,
-                                       prefix=colorstr(f'{task}: '))[0]
+                                       workers=workers, prefix=colorstr(f'{task}: '))[0]
 
     seen = 0
     confusion_matrix = ConfusionMatrix(nc=nc)
     parser.add_argument('--iou-thres', type=float, default=0.6, help='NMS IoU threshold')
     parser.add_argument('--task', default='val', help='train, val, test, speed or study')
     parser.add_argument('--device', default='', help='cuda device, i.e. 0 or 0,1,2,3 or cpu')
+    parser.add_argument('--workers', type=int, default=8, help='max dataloader workers (per RANK in DDP mode)')
     parser.add_argument('--single-cls', action='store_true', help='treat as single-class dataset')
     parser.add_argument('--augment', action='store_true', help='augmented inference')
     parser.add_argument('--verbose', action='store_true', help='report mAP by class')