ultralytics 8.0.18 new python callbacks and minor fixes (#580)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jeroen Rombouts <36196499+jarombouts@users.noreply.github.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/nn/autobackend.py

@@ -222,7 +222,8 @@ class AutoBackend(nn.Module):
             nhwc = model.runtime.startswith("tensorflow")
             '''
         else:
-            raise NotImplementedError(f'ERROR: {w} is not a supported format')
+            raise NotImplementedError(f"ERROR: '{w}' is not a supported format. For supported formats see "
+                                      f"https://docs.ultralytics.com/reference/nn/")
 
         # class names
         if 'names' not in locals():