[docs] use device-agnostic API instead of cuda (#34913)

add device-agnostic API

Signed-off-by: Lin, Fanli <fanli.lin@intel.com>

docs/source/en/perplexity.md

@@ -73,8 +73,9 @@ Let's demonstrate this process with GPT-2.
 
 ```python
 from transformers import GPT2LMHeadModel, GPT2TokenizerFast
+from accelerate.test_utils.testing import get_backend
 
-device = "cuda"
+device, _, _ = get_backend() # automatically detects the underlying device type (CUDA, CPU, XPU, MPS, etc.)
 model_id = "openai-community/gpt2-large"
 model = GPT2LMHeadModel.from_pretrained(model_id).to(device)
 tokenizer = GPT2TokenizerFast.from_pretrained(model_id)