Fix a shape annotation and typos in `mamba` slow forward
Author: vasqu
Pull request: yes

# What does this PR do?
It only addresses typos and a wrong shape annotation in the comments of `mamba`'s slow forward call. There's no change in the logic or anything.

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Comments:
Yup, same typo. The shape annotation is correct tho.

Another thing I've noticed in `Jamba` are these line;
https://github.com/huggingface/transformers/blob/5962d62bac850cd01ee830ffba880469338c96fd/src/transformers/models/jamba/modeling_jamba.py#L916-L920

If you remember my issue from the past ( #29526 ), I've added something similar but not differentiating between `training` and `eval`. Might be worth to change in base mamba as well. 

Down to change, but with a bench / something that shows it does produce improvements!