Why don't use gpu

## install
pip install llama-cpp-python \
--extra-index-url https://abetlen.github.io/llama-cpp-python/whl/cu121 --no-cache-dir

## code
from llama_cpp import Llama

model_path="/root/model/Llama3.1-8B-Chinese-Chat-gguf/Llama3.1-8B-Chinese-Chat.Q3_K_M.gguf"

model_kwargs = {
"n_ctx":8192, # Context length to use
"n_threads":4, # Number of CPU threads to use
"n_gpu_layers": 20 ,# Number of model layers to offload to GPU. Set to 0 if only using CPU
}

llm = Llama(model_path=model_path, **model_kwargs)

generation_kwargs = {
"max_tokens":2000, # Max number of new tokens to generate
# "stop":["<|endoftext|>", ""], # Text sequences to stop generation on
"echo":False, # Echo the prompt in the output
"top_k":3 # This is essentially greedy decoding, since the model will always return the highest-probability token. Set this value > 1 for sampling decoding
}

def chat(messages):
res = llm.create_chat_completion(
messages=messages
)

print(res['choices'][0]['message']['content'])

if __name__ == '__main__':
while True:
prompt = input()
messages = [
{"role": "user","content": prompt}
]
chat(messages)

questions:
why do not use gpu?