[Platform] add dispatch key (#17)

### What this PR does / why we need it?
Add dispatch key for NPU, so that the log could be print correctly.

Now
```
executor_base.py:110] # CPU blocks: 220478, # CPU blocks: 21845
```

After this pr
```
executor_base.py:110] # NPU blocks: 220478, # CPU blocks: 21845
```

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested?
CI passed and log printed as above

Signed-off-by: MengqingCao <cmq0113@163.com>

Author: MengqingCao

vllm_ascend/platform.py

@@ -57,6 +57,7 @@ class NPUPlatform(Platform):
     simple_compile_backend: str = "npu"
     ray_device_key: str = "NPU"
     device_control_env_var: str = "ASCEND_RT_VISIBLE_DEVICES"
+    dispatch_key: str = "PrivateUse1"
 
     supported_quantization: list[str] = ["ascend"]