[`Mamba2`] Move dt calculations to kernel
Author: vasqu
Pull request: yes
Labels: name: run-slow

# What does this PR do?
Moves calculations of `time_step` to the kernel instead of using torch's softplus directly. It seems like it influences the end result even more than expected, see https://github.com/sustcsonglin/flash-linear-attention/issues/63 for reference (2. point made there). 

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Comments:
Maybe for clarification the referenced repo's mamba2 is mostly a mirror of this one.

Thanks for the PR! Very interesting, not opposed but what exactly changes between the in-kernel and out-of-kernel computation results? Train mode vs eval mode yield different results? In that case we should also add it as a minimal test specific for mamba2!

Just seems like that it's more aligned with the completely fused kernel then, or rather the numerical differences are minimized. Tbh it's a huge nit in and of itself :D 

I can add a slow test that requires a GPU and the kernels; would be a copy paste of the test in the repo. 

I'll write the test tomorrow and ping you then; I don't have much time this evening. 

Would be ready but ig waiting for #33560 to be merged seems appropriate for now :eyes: 
cc @amyeroberts 

Does it need a new decorator to check if the kernels are available or is it directly included in the slow runs? Might need an updated image if that's not the case. @molbap 

Nope, there's no such decorator! Slow run ought to do it :) 

Good to go now :) #33567 is to be expected
cc @amyeroberts (sry for the constant pinging)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33520). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@vasqu Pings are always welcome - it makes sure things aren't lost in the notification inbox :)