Threadpool fixes and CLI improvements (#550)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ayush Chaurasia <ayush.chaurarsia@gmail.com>

ultralytics/yolo/data/dataloaders/v5loader.py

@@ -13,7 +13,7 @@ import random
 import shutil
 import time
 from itertools import repeat
-from multiprocessing.pool import Pool, ThreadPool
+from multiprocessing.pool import ThreadPool
 from pathlib import Path
 from threading import Thread
 from urllib.parse import urlparse
@@ -580,7 +580,7 @@ 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
-            with (Pool if n > 10000 else ThreadPool)(NUM_THREADS) as pool:
+            with ThreadPool(NUM_THREADS) as pool:
                 results = pool.imap(fcn, range(n))
                 pbar = tqdm(enumerate(results), total=n, bar_format=TQDM_BAR_FORMAT, disable=LOCAL_RANK > 0)
                 for i, x in pbar:
@@ -1150,7 +1150,7 @@ class HUBDatasetStats():
             dataset = LoadImagesAndLabels(self.data[split])  # load dataset
             desc = f'{split} images'
             total = dataset.n
-            with (Pool if total > 10000 else ThreadPool)(NUM_THREADS) as pool:
+            with ThreadPool(NUM_THREADS) as pool:
                 for _ in tqdm(pool.imap(self._hub_ops, dataset.im_files), total=total, desc=desc):
                     pass
         print(f'Done. All images saved to {self.im_dir}')