Update `check_file()` avoid repeat URL downloads (#5526)

utils/general.py

@@ -338,9 +338,12 @@ def check_file(file, suffix=''):
     elif file.startswith(('http:/', 'https:/')):  # download
         url = str(Path(file)).replace(':/', '://')  # Pathlib turns :// -> :/
         file = Path(urllib.parse.unquote(file).split('?')[0]).name  # '%2F' to '/', split https://url.com/file.txt?auth
-        print(f'Downloading {url} to {file}...')
-        torch.hub.download_url_to_file(url, file)
-        assert Path(file).exists() and Path(file).stat().st_size > 0, f'File download failed: {url}'  # check
+        if Path(file).is_file():
+            print(f'Found {url} locally at {file}')  # file already exists
+        else:
+            print(f'Downloading {url} to {file}...')
+            torch.hub.download_url_to_file(url, file)
+            assert Path(file).exists() and Path(file).stat().st_size > 0, f'File download failed: {url}'  # check
         return file
     else:  # search
         files = []