Mamba `slow_forward` gradient fix
Author: vasqu
Pull request: yes

# What does this PR do?
- Fixes the gradient issue with mamba in the `slow_forward` by cloning the initial `ssm_state` obtained via the cache.
- Fixes typo in mamba test.
- Adds unused `create_and_check_forward_and_backwards` to the tests.
- Adds a `slow_forward` test (`test_mamba_cached_slow_forward_and_backwards`) checking if the cache doesn't break anything in the backward pass.

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Fixes #29514 and duplicate #29526


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
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      to it if that's the case. See #29526
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- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@amyeroberts & @ArthurZucker 

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam
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- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
Hi @vasqu and @amyeroberts! is this PR needed in case one wants to run Mamba (training, PEFT, TRL, generation/inference) within an environment having installed causal-conv1d and mamba compiled from sources and using GPU? I was wondering whether to wait for this to be fixed or just install the last commit in main. Thanks!

Hi @javiermcebrian, you won't need this fix if you have the `mamba-ssm` and `causal-conv1d` packages installed (including a GPU ofc). In case the packages are not recognized, you will get a warning and then this fix might be of interest. 

I can only speak for training and inference/generation, but for a private project (the "fast" variant), it worked fine.

All looks good to me - thanks for adding this! 

Let's just get a final review from @ArthurZucker to confirm behaviour is as desired for mamba and there isn't anything else that needs to be added 

Reviewing now!