Fix: Mamba2 generation mismatch between input_ids and inputs_embeds
Author: vasqu
Pull request: yes

# What does this PR do?
Fixes the failing generation test in mamba2 comparing the generation between passing input ids or passing the embeds directly. The cause of the mismatch was a false calculation of `past_len` which can be solved by changing the if check slightly.

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Fixes #32555


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Comments:
Lmk if there's a better way to do this but I think this should be ok. At least the cause of the issue is apparent now.

That's a nice fix, thanks! I wonder if we can use `cache_position` instead, it would be cleaner/more aligned with the rest of the lib, other that that LGTM

@ArthurZucker Isn't it also tested for all mamba-related models already? I've run the mamba2 tests locally for example and the test you linked was included in them (can also be seen in the CI of this PR over [here](https://app.circleci.com/pipelines/github/huggingface/transformers/100809/workflows/131d69f6-1861-451e-8a2e-747aa3e78b6f/jobs/1343122?invite=true#step-113-14389_119)). 

Maybe I've misunderstood something :sweat_smile: Could you clarify the test?

You are right I forgot that it was failing ever since we added it!


Thanks @vasqu for maintaining mamba2 😉 

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