[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>

examples/offline_disaggregated_prefill_npu.py

@@ -0,0 +1,140 @@
+#
+# Copyright (c) 2025 Huawei Technologies Co., Ltd. All Rights Reserved.
+# This file is a part of the vllm-ascend project.
+# Adapted from vllm-project/vllm/examples/offline_inference/basic.py
+# Copyright 2023 The vLLM team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+import multiprocessing as mp
+import os
+import time
+from multiprocessing import Event, Process
+
+
+def clean_up():
+    import gc
+
+    import torch
+    from vllm.distributed.parallel_state import (
+        destroy_distributed_environment, destroy_model_parallel)
+    destroy_model_parallel()
+    destroy_distributed_environment()
+    gc.collect()
+    torch.npu.empty_cache()
+
+
+def run_prefill(prefill_done, process_close):
+    os.environ["ASCEND_RT_VISIBLE_DEVICES"] = "0,1"
+
+    from vllm import LLM, SamplingParams
+    from vllm.config import KVTransferConfig
+
+    prompts = [
+        "Hello, how are you today?", "Hi, what is your name?",
+        "Tell me a very long story.", "what is your favourite book?"
+    ]
+    sampling_params = SamplingParams(temperature=0, top_p=0.95, max_tokens=1)
+
+    ktc = KVTransferConfig.from_cli(
+        '{"kv_connector":"AscendHcclConnector","kv_buffer_device":"npu","kv_role":"kv_producer", "kv_parallel_size":2}'
+    )
+
+    # Set NPU memory utilization to 0.8
+    llm = LLM(model="deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B",
+              kv_transfer_config=ktc,
+              max_model_len=2000,
+              gpu_memory_utilization=0.8,
+              tensor_parallel_size=2)
+
+    llm.generate(prompts, sampling_params)
+    print("Prefill node is finished.")
+    prefill_done.set()
+
+    # To keep the prefill node running in case the decode node is not done
+    # otherwise, the script might exit prematurely, causing incomplete decoding.
+    try:
+        while not process_close.is_set():
+            time.sleep(1)
+    except KeyboardInterrupt:
+        print("Script stopped by user.")
+    finally:
+        print("Cleanup prefill resources")
+        del llm
+        clean_up()
+
+
+def run_decode(prefill_done):
+    os.environ["ASCEND_RT_VISIBLE_DEVICES"] = "2,3"
+
+    from vllm import LLM, SamplingParams
+    from vllm.config import KVTransferConfig
+
+    prompts = [
+        "Hello, how are you today?", "Hi, what is your name?",
+        "Tell me a very long story.", "what is your favourite book?"
+    ]
+    sampling_params = SamplingParams(temperature=0, top_p=0.95)
+
+    ktc = KVTransferConfig.from_cli(
+        '{"kv_connector":"AscendHcclConnector","kv_buffer_device":"npu","kv_role":"kv_consumer","kv_parallel_size":2}'
+    )
+
+    llm = LLM(model="deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B",
+              kv_transfer_config=ktc,
+              max_model_len=2000,
+              gpu_memory_utilization=0.8,
+              tensor_parallel_size=2)
+
+    # Wait for the producer to start the consumer
+    print("Waiting for prefill node to finish...")
+    prefill_done.wait()
+
+    # At this point when the prefill_done is set, the kv-cache should have been
+    # transferred to this decode node, so we can start decoding.
+    outputs = llm.generate(prompts, sampling_params)
+    for output in outputs:
+        prompt = output.prompt
+        generated_text = output.outputs[0].text
+        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+
+    del llm
+    clean_up()
+
+
+if __name__ == "__main__":
+    mp.get_context('spawn')
+
+    prefill_done = Event()
+    process_close = Event()
+    prefill_process = Process(target=run_prefill,
+                              args=(
+                                  prefill_done,
+                                  process_close,
+                              ))
+    decode_process = Process(target=run_decode, args=(prefill_done, ))
+
+    # Start prefill node
+    prefill_process.start()
+
+    # Start decode node
+    decode_process.start()
+
+    # Terminate the prefill node when decode is finished
+    decode_process.join()
+
+    # Terminate prefill process
+    process_close.set()
+    prefill_process.join()
+    prefill_process.terminate()
+    print("All process done!")