[ModelLoader][Feature] Add rfork support for fast model loading (#7392)

### What this PR does / why we need it?
Support an new load format: RFORK

For implementation details of this feature, please refer to #7441


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

add an new options for load-format: rfork

e.g.
```bash
vllm serve /workspace/models/Qwen3-8B --load-format rfork
```

### How was this patch tested?

- vLLM version: v0.17.0
- vLLM main:
4034c3d32e

Signed-off-by: Marck <1412354149@qq.com>

vllm_ascend/model_loader/rfork/seed_server.py

@@ -0,0 +1,126 @@
+#
+# Copyright (c) 2026 Huawei Technologies Co., Ltd. All Rights Reserved.
+#
+# 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 queue
+import socket
+import threading
+import time
+from http import HTTPStatus
+
+import requests
+import uvicorn
+from fastapi import FastAPI
+from fastapi.responses import Response
+from vllm.logger import logger
+
+
+def start_fastapi_server(
+    port_queue: queue.Queue[int],
+    local_seed_key,
+    info,
+):
+    logger.info("[RFork Seed] Preparing socket with dynamic port...")
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(("0.0.0.0", 0))
+    _, port = sock.getsockname()
+    logger.info("[RFork Seed] Assigned dynamic port: %s", port)
+
+    app = FastAPI()
+
+    @app.get("/get_rfork_transfer_engine_info")
+    def get_rfork_transfer_engine_info(seed_key: str):
+        if seed_key == local_seed_key:
+            return {"rfork_transfer_engine_info": info}
+        return {"rfork_transfer_engine_info": None}
+
+    @app.get("/rfork_fetch_seed")
+    def rfork_fetch_seed():
+        return {"status": "ok"}
+
+    @app.get("/health_check_with_key")
+    def health_check_with_key(seed_key: str):
+        if seed_key == local_seed_key:
+            return Response(status_code=HTTPStatus.OK)
+        return Response(status_code=HTTPStatus.BAD_REQUEST)
+
+    config = uvicorn.Config(app, host=None, port=None, log_level="warning")
+    server = uvicorn.Server(config)
+
+    try:
+        port_queue.put(port)
+    except Exception as e:
+        logger.error("[RFork Seed] Failed to send port via queue: %s", e)
+        sock.close()
+        return
+
+    logger.info("[RFork Seed] FastAPI server starting on port %s...", port)
+    server.run(sockets=[sock])
+    sock.close()
+
+
+def start_rfork_server(local_seed_key, rfork_transfer_engine_info, health_timeout_sec: float = 30.0) -> int:
+    port_queue: queue.Queue[int] = queue.Queue()
+    process = threading.Thread(
+        target=start_fastapi_server,
+        args=(port_queue, local_seed_key, rfork_transfer_engine_info),
+        daemon=True,
+    )
+    process.start()
+
+    try:
+        port = port_queue.get(timeout=15)
+        if port == -1:
+            raise RuntimeError("Child process failed to start server")
+    except Exception as e:
+        logger.error("[RFork Seed] start server error: %s", e)
+        return -1
+
+    deadline = time.time() + health_timeout_sec
+    healthy = False
+    retry_count = 0
+    last_error = None
+    while time.time() < deadline:
+        time.sleep(0.01)
+        url = f"http://127.0.0.1:{port}/health_check_with_key"
+        try:
+            response = requests.get(
+                url,
+                params={"seed_key": local_seed_key},
+                timeout=10,
+            )
+            if response.status_code == 200:
+                healthy = True
+                break
+            last_error = f"unexpected status code {response.status_code} from health check"
+        except Exception as e:
+            last_error = str(e)
+        retry_count += 1
+    if healthy:
+        if retry_count > 1:
+            logger.info(
+                "[RFork Seed] health check passed after %d retries for port %s",
+                retry_count - 1,
+                port,
+            )
+        return port
+    logger.error(
+        "[RFork Seed] health check timed out after %.1fs for port %s, last error: %s",
+        health_timeout_sec,
+        port,
+        last_error,
+    )
+    return -1