Fix load balancing loss func for mixtral
Author: liangxuZhang
Pull request: yes

# What does this PR do?

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Fixes #28255


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@ArthurZucker and @younesbelkada
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Comments:
How does this differ from https://github.com/huggingface/transformers/pull/28115 ?

What is the impact of this issue on Mixtral training?  Will this fix conceivability improve the quality of training?  Is it likely that previous Mixtral trainings are not as good as they could be?

It seems like an important issue for those working with Mixtral that has been waiting on merge approval for a while.


I personally finds the loss to be much lower with the new implementation. But I wasn't sure if it has to do with the (num_experts**2) instead of just N. I'm pretty sure this is an error on original mixtral side. So far still waiting for the training result on new implemented balance loss to finish. Deepspeed also has an implementation of top-2 which we might be able to reference.

#28255 has information that could help, I am down to merge this for the release planned this week, just the comments that need to be adressed cc @liangxuZhang do you need help to finish this? 

> #28255 has information that could help, I am down to merge this for the release planned this week, just the comments that need to be adressed cc @liangxuZhang do you need help to finish this?

@ArthurZucker LGTM. The new implementation is correct and concise, and I've made a new commit. In #28255, maybe we can have a deep discuss whether to concatenate gate logits of all layers.

Alright! Pretty sure the math shows it's equivalent to compute on individual layers then sum vs concate and compute, but let's merge this for now !

> Thanks! Failing test seems unrelated let's just rebase on main

@ArthurZucker I've just rebase on the main branch, but I'm not sure if I'm doing it right. Please tell me what else I need to do

@liangxuZhang @ArthurZucker opinions about https://github.com/huggingface/transformers/pull/28403 ? It looks complementary to this PR

Something like `git pull upstream main` if the remote is `upstream`, the exotic CI was fixed on main! I'll merge without it 

Thanks a lot @liangxuZhang for this fix! 🤗 

great！

Impressive work!