replace the device control env var as placeholder

Signed-off-by: ganyi <pleaplusone.gy@gmail.com>

Author: ganyi1996ppo

vllm_ascend/platform.py

@@ -59,7 +59,7 @@ class NPUPlatform(Platform):
     simple_compile_backend: str = "eager"  # Disable torch.compile()
     ray_device_key: str = "NPU"
     # device_control_env_var: str = "ASCEND_RT_VISIBLE_DEVICES"
-    device_control_env_var: str = ""
+    device_control_env_var: str = "plcaceholder"
     dispatch_key: str = "PrivateUse1"
 
     supported_quantization: list[str] = [ASCEND_QUATIZATION_METHOD]