Add Code Llama Support and Fix empty system prompt for llama 2

[woshiyyya]

Why are these changes needed?

Integrate codellama/CodeLlama-7b-Instruct-hf into FastChat. (Tested with the 7b model, but should also work for the larger models.)

(Minor fix: Handled the case when system_prompt == "" for SeparatorStyle.LLAMA2.)

Example usage:

# Run with system message
python3 -m fastchat.serve.cli --model-path codellama/CodeLlama-7b-Instruct-hf --conv-system-msg "Provide answers in Javascript"

# Run without system message
python3 -m fastchat.serve.cli --model-path codellama/CodeLlama-7b-Instruct-hf

Reference: Llama 2 learns to code

Related issue number (if applicable)

Closes #2309

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).