[`Llava`] Fix convert_llava_weights_to_hf.py script
Author: isaac-vidas
Pull request: yes

Fix call to `tokenizer.add_tokens` in `convert_llava_weights_to_hf.py` and `convert_vipllava_weights_to_hf.py` scripts.

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Comments:
@isaac-vidas would you be happy to address the same fix for Vip-Llava? https://github.com/huggingface/transformers/blob/main/src/transformers/models/vipllava/convert_vipllava_weights_to_hf.py#L61

Thanks for the review @younesbelkada!
I added the fix you mentioned, if you can review again please 😄 

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