add a callback hook right before the optimizer step

[winglian]

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

Fixes # (issue)

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[LysandreJik]

Thanks for the PR @winglian ! Pinging @muellerzr for a quick review

[muellerzr]

pre_optimizer_step seems like a good enough naming convention to it with what we have happening around IMO. Thanks!

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[LysandreJik]

🚀