Error in usage example for gctf.centralized_gradients_for_optimizer

[Rishit-dagli]

I noticed that the docstrings for gctf.centralized_gradients_for_optimizer have an error in the example usage section. The example creates an Adam optimizer instance and saves it to opt however the centralized_gradients_for_optimizer is applied on optimizer which ideally does not exist and running the example would result in an error.