imgsz warning fix, download function consolidation (#681)

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

ultralytics/yolo/data/base.py

@@ -120,7 +120,8 @@ class BaseDataset(Dataset):
                 im = np.load(fn)
             else:  # read image
                 im = cv2.imread(f)  # BGR
-                assert im is not None, f"Image Not Found {f}"
+                if im is None:
+                    raise FileNotFoundError(f"Image Not Found {f}")
             h0, w0 = im.shape[:2]  # orig hw
             r = self.imgsz / max(h0, w0)  # ratio
             if r != 1:  # if sizes are not equal