Fix missing labels after albumentations (ultralytics#4455)

* fix missing labels after augmentation

* Update datasets.py

Cleanup

Co-authored-by: Huu Quan <huuquan@HuuQuans-MacBook.local>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/datasets.py

@@ -568,6 +568,7 @@ def __getitem__(self, index):
         if self.augment:
             # Albumentations
             img, labels = self.albumentations(img, labels)
+            nl = len(labels) # update after albumentations
 
             # HSV color-space
             augment_hsv(img, hgain=hyp['hsv_h'], sgain=hyp['hsv_s'], vgain=hyp['hsv_v'])