update the kv cache code to be compatible with latest transformers repository

[jerryliang24]

Thanks for your great work! I've read the code and implemented some of the experiments myself. However, I value the great potential of coconut applying to many modern models (Qwen3, Mistral or even DeepSeek series)! So I modified the part regarding kv cache to make the code compatible with latest transformers repository, and I hope this would be helpful.

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