[Feature] Add PD separation feature (#432)

### What this PR does / why we need it?
Adapt Disaggregated Prefill feature onto Ascend device

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

The test usage has been provided alongwith the PR, in
examples/offline_disaggregated_prefill_npu.py
To run it, do this
```
export PROMPT_DEVICE_ID=0,1
export DECODE_DEVICE_ID=2,3
python examples/offline_disaggregated_prefill_npu.py
```

---------

Signed-off-by: ZihuiQian <qianzihui@huawei.com>
Co-authored-by: ZihuiQian <qianzihui@huawei.com>

vllm_ascend/platform.py

@@ -150,7 +150,7 @@ class NPUPlatform(Platform):
 
     @classmethod
     def get_device_communicator_cls(cls) -> str:
-        return "vllm_ascend.communicator.NPUCommunicator"
+        return "vllm_ascend.distributed.communicator.NPUCommunicator"
 
     @classmethod
     def is_pin_memory_available(cls):