Add tqdm pbar.close() (ultralytics#2644)

When using tqdm, sometimes it can't print in one line and roll to next line.

(cherry picked from commit 1e8ab3f)

utils/datasets.py

@@ -443,7 +443,8 @@ def __init__(self, path, img_size=640, batch_size=16, augment=False, hyp=None, r
                 self.imgs[i], self.img_hw0[i], self.img_hw[i] = x  # img, hw_original, hw_resized = load_image(self, i)
                 gb += self.imgs[i].nbytes
                 pbar.desc = f'{prefix}Caching images ({gb / 1E9:.1f}GB)'
-
+            pbar.close()
+
     def cache_labels(self, path=Path('./labels.cache'), prefix=''):
         # Cache dataset labels, check images and read shapes
         x = {}  # dict
@@ -487,7 +488,8 @@ def cache_labels(self, path=Path('./labels.cache'), prefix=''):
 
             pbar.desc = f"{prefix}Scanning '{path.parent / path.stem}' images and labels... " \
                         f"{nf} found, {nm} missing, {ne} empty, {nc} corrupted"
-
+        pbar.close()
+
         if nf == 0:
             print(f'{prefix}WARNING: No labels found in {path}. See {help_url}')