Add on_after_backwards callback option
🚀 Feature
Add an on_after_backward callback option.
Motivation
Currently there are no callbacks for after the backwards step so non-essential code like logging gradients clutters the LightningModule.
Pitch
Expand the callback options to include a hook for on_after_backward() to execute callbacks immediately after the backwards pass
Alternatives
Sticking my non-essential gradient logging code in my LightningModule