Fix: Mamba2 `norm_before_gate` usage
Author: vasqu
Pull request: yes

# What does this PR do?
Fixes the default value for norm_before_gate (False) in the default config of mamba2. Additionally, implemented the other variation with norm_before_gate = True. Currently, this only affects continuous training from codestral on the fast path with the fused kernel: https://github.com/huggingface/transformers/blob/20a04497a86dc79adb8a93e31325f25255d317e1/src/transformers/models/mamba2/modeling_mamba2.py#L334-L353 
Discovered in https://github.com/huggingface/transformers/pull/32580#discussion_r1712661960

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Comments:
Tbh, we could also just remove `norm_before_gate` in the config and only allow the `False` path. I don't mind either. 

Will rebase later when the other fix(es) are in main :smile:  

Hmm, there are currently two problems in the current implementation though:
1. The `norm_before_gate` flag in the config was incorrectly set to True but the implementation follows the path as if it were False. This is problematic as it does not suggest so and enables incorrect codestral training via the fast path (see the initial description and the code snippet - direct passing of the flag).
2. Now that the flag exists, it would be weird that it only supports one path but suggests otherwise.

If I read it correctly, would you prefer removing this flag in the config and adjusting the code to only follow one path (i.e. `norm_before_gate=False`)?

> If I read it correctly, would you prefer removing this flag in the config and adjusting the code to only follow one path (i.e. norm_before_gate=False)?

yes! 🤗 this would be less confusing IMO! 

@ArthurZucker Should be good now :eyes: 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32686). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

I guess the renaming of the PR is too late now 😓 

No worries 🤗