missing scaler.scale in manual_backward?
🐛 Bug
When using manual_backward and precision=16, an exception is raised:
RuntimeError: unscale_() has already been called on this optimizer since the last update().
As far as I can see, the scaler is actually never used to scale the loss during manual_backward and in the example here it is not mentioned that one has to do it before.
Also, I cannot find any 'update' call
colab model to reproduce:
https://colab.research.google.com/drive/1n7-NxC0IJ3gYJGZLsoeOtu-oQc3I8UjF?usp=sharing