Support LoRA in Completion API

[bjmsong]

Motivation

relate to this issue

Usage

python -m sglang.launch_server --model-path {model_path} --lora-paths lora0={lora_path} --disable-radix --disable-cuda-graph --max-loras-per-batch=4

python -m sglang.bench_serving --backend sglang-oai --model {model_path} --num-prompts 1 --lora-name lora0

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.