Add warning about --dtype arg in reference_hf.py.

[aw632]

When I was following this guide, I noticed that these commands gave different outputs:

python3 /sgl-workspace/sglang/python/sglang/bench_one_batch.py --correct --model meta-llama/Llama-3.2-3B-Instruct
python3 /sgl-workspace/sglang/scripts/playground/reference_hf.py --model-path meta-llama/Llama-3.2-3B-Instruct --model-type text

It turns out that reference_hf.py was autocasting to float16 silently, which was causing this issue. I added this comment in case others come across the same issue.