SnapKV_Cache support added
Author: shirinyamani
Pull request: yes

# What does this PR do?

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This PR adds the implementation of [SnapKV Cache paper](https://arxiv.org/abs/2404.14469) to [cache_utils.py](https://github.com/huggingface/transformers/blob/main/src/transformers/cache_utils.py) file. Also adds the changes in flash_attention2 in llama_modeling to reflect the initialization of the SnapKV cache.


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Comments:
Pinging the cache gang @zucchini-nlp @ArthurZucker @gante

@ArthurZucker Thanks alot for your response! So do you think if I remove the modeling changes then the `SnapKV` class is fine to be added to the `cache_utils.py` ? , couldn't find the `cache_positions` you are referring to in the files (cache n llama)!

Yes ! https://github.com/huggingface/transformers/blob/0ade1caa356dce6b70ef8293addeb0898f177206/src/transformers/models/llama/modeling_llama.py#L282 cache positions are here. 
Overall we should not need modeling changes! 

But with #35235 you can maybe find a better way to support this by creating a new `attn_implementation` and add it to the mapping? 🤗