Add support for Vigogne models

[bofenghuang]

Why are these changes needed?

Hi!

Thank you for your excellent work! I'm excited to contribute by adding support for the Vigogne models. Vigogne represents a collection of 🇫🇷 French LLMs specifically fine-tuned for instruction-following and interactive chat.

I have followed the steps outlined in the how-to-support-a-new-model section and managed to run the model using the following commands:

# For Vigogne-Instruct models
# python3 -m fastchat.serve.cli --model bofenghuang/vigogne-2-7b-instruct

# For Vigogne-Chat models
python3 -m fastchat.serve.cli --model bofenghuang/vigogne-2-7b-chat

Thanks in advance!

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Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).