Fix potential memory leak for cache ram (#13525)

utils/dataloaders.py

@@ -688,7 +688,8 @@ class LoadImagesAndLabels(Dataset):
             b, gb = 0, 1 << 30  # bytes of cached images, bytes per gigabytes
             self.im_hw0, self.im_hw = [None] * n, [None] * n
             fcn = self.cache_images_to_disk if cache_images == "disk" else self.load_image
-            results = ThreadPool(NUM_THREADS).imap(lambda i: (i, fcn(i)), self.indices)
+            with ThreadPool(NUM_THREADS) as pool:
+                results = pool.imap(lambda i: (i, fcn(i)), self.indices)
                 pbar = tqdm(results, total=len(self.indices), bar_format=TQDM_BAR_FORMAT, disable=LOCAL_RANK > 0)
                 for i, x in pbar:
                     if cache_images == "disk":