Add a converter from mamba_ssm -> huggingface mamba
Author: byi8220
Pull request: yes

# What does this PR do?
Fixes #29631

This PR creates a new command line script `convert_mamba_ssm_checkpoint_to_pytorch.py` which converts model checkpoints created by the [state-spaces/mamba](https://github.com/state-spaces/mamba/tree/main) repo into a Huggingface `MambaForCausalLM` model.

The intended usage of this script is:
```
python src/transformers/models/mamba/convert_mamba_ssm_checkpoint_to_pytorch.py \
--mamba_checkpoint_file=path/to/pytorch_model.bin \
--config_json_file=/path/to/ssm_config.json \
--output_dir=/path/to/out/dir
```

This script has a dependency on the `mamba_ssm` package.

### Testing

A validation pass is performed before exporting the model.

## Before submitting
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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.

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- text models: @ArthurZucker and @younesbelkada
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Library:

- flax: @sanchit-gandhi
- generate: @gante
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Comments:
Thanks for opening this PR @byi8220 ! Let us know when it's ready for review 🤗 

Thanks, I think it should be reviewable now?

Addressed nits.

Since there's no unit test for this anymore just did one last sanity test. Works on my machine

1. My script used to spot test the conversion (https://gist.github.com/byi8220/9b44a5f6c6c2c7533704801478f1760a) passes for the 130m and 790m models. My GPU doesn't have enough memory to run the larger ones.
2. Downloading the weights and config of a [130m model online](https://huggingface.co/state-spaces/mamba-130m/tree/main) and running the command `python src/transformers/models/mamba/convert_mamba_ssm_checkpoint_to_pytorch.py --mamba_checkpoint_file=/tmp/scratch/pytorch_model.bin --config_json_file=/tmp/scratch/config.json --output_dir=/tmp/scratch/out` appears to function e2e.


Awesome work - thanks again for adding this and for running some sanity checks!