add a callback hook right before the optimizer step
Author: winglian
Pull request: yes

# What does this PR do?

grokfast paper: https://arxiv.org/abs/2405.20233
see: https://github.com/ironjr/grokfast 
grokfast augments the optimizer by modifying gradients right before the optimizer step
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Comments:
Thanks for the PR @winglian ! Pinging @muellerzr for a quick review

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