Fix sliding window attention used in Gemma2FlashAttention2
Author: brcps12
Pull request: yes
Labels: name: run-slow

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Currently, implementation of the sliding window in the `Gemma2FlashAttention2` module has issues. Specifically, when applying the sliding window, [the attention mask is sliced](https://github.com/huggingface/transformers/blob/f5cdbf6e546784639c2d8f72420198138d2a2a13/src/transformers/models/gemma2/modeling_gemma2.py#L575). This can lead to problems if the sequence length exceeds the sliding window size.

Instead, I use the `window_size` parameter supported by Flash Attention 2. This ensures the sliding window is applied correctly, regardless of sequence length.


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Comments:
cc @ArthurZucker 

I committed with the message.

Additionally, regardless of whether the tests pass, an error occurs when generating sentences with the current implementation of `gemma2 + flash_attn2`. That likely needs to be fixed

I think it could be implemented by referring to the [MistralFlashAttention2](https://github.com/huggingface/transformers/blob/main/src/transformers/models/mistral/modeling_mistral.py#L320-L347) code.

Should I make these changes in this PR?


Done.

Also I added a condition `past_key_value is not None` for decoding phase.

Thanks @brcps12 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32522). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

It seems that @brcps12 submitted this PR, but his name was unfortunately not mentioned in the release notes. Could we update the release notes to include his contribution?

yeah for sure

Sorry about that, done!