Don't raise in Imagenette download

Author: NicolasHug

torchvision/datasets/imagenette.py

@@ -81,10 +81,7 @@ def _check_exists(self) -> bool:
 
     def _download(self):
         if self._check_exists():
-            raise RuntimeError(
-                f"The directory {self._size_root} already exists. "
-                f"If you want to re-download or re-extract the images, delete the directory."
-            )
+            return
 
         download_and_extract_archive(self._url, self.root, md5=self._md5)