[Misc] Add a model loader that utilizes HCCL for weight loading (#2888)

### What this PR does / why we need it?

This PR introduces a new model loader called Netloader, which leverages
high-bandwidth P2P direct transfer between NPU cards to achieve weight
loading. Netloader is implemented as a plugin through the newly added
'register_model_loader' function in vLLM 0.10. It facilitates the
process of weight loading by sending weights from a pre-loaded model
(server) to an empty model of a newly started instance (client). The
server operates concurrently with normal inference tasks through
sub-threads and the 'stateless_init_torch_distributed_process_group' in
vLLM. The client initiates a transfer request after verifying that the
model and partitioning method are the same as the server's, and uses
HCCL's collective communication (send/recv) to load the weights in the
order they are stored in the model.

Application Scenarios:
1. Significantly Reduces Inference Instance Startup Time By reusing the
weights of already loaded instances and performing high-speed transfers
directly between computing cards, this method reduces model loading
latency compared to traditional remote/local pull methods.
2. Reduces Network and Storage Pressure Avoids the need to repeatedly
download weight files from remote repositories, reducing the impact on
centralized storage and network traffic, thereby enhancing overall
system stability and service quality.
3. Improves Resource Utilization and Reduces Costs Accelerating the
loading process reduces reliance on redundant computing pools, allowing
computing resources to be elastically scaled and reclaimed as needed.
4. Enhances Business Continuity and High Availability In fault recovery
scenarios, new instances can quickly take over existing services,
avoiding prolonged business interruptions and improving the system's
high availability and user experience.

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

Netloader utilizes the existing --load-format=netloader and
--model-loader-extra-config to be activated. The
model-loader-extra-config needs to be input as a JSON string (as it is
now)

Afterwards, you can check whether the outputs for the same sentence are
consistent when the temperature is set to 0.

Signed-off-by: destinysky <kangrui10@126.com>

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: destinysky <kangrui10@126.com>

tests/ut/model_loader/netloader/test_netloader_utils.py

@@ -0,0 +1,61 @@
+#
+# Copyright (c) 2025 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 os
+import tempfile
+from unittest.mock import patch
+
+import pytest
+
+from vllm_ascend.model_loader.netloader.utils import (find_free_port,
+                                                      is_valid_path_prefix)
+
+
+def test_find_free_port():
+    port = find_free_port()
+    assert isinstance(port, int)
+    assert port > 0
+
+
+def test_is_valid_path_prefix_empty():
+    assert not is_valid_path_prefix('')
+
+
+def test_is_valid_path_prefixIllegal_characters():
+    assert not is_valid_path_prefix('test<>:"|?*')
+
+
+def test_is_valid_path_prefixRelative_path():
+    assert is_valid_path_prefix('test')
+
+
+def test_is_valid_path_prefixAbsolute_path():
+    with tempfile.TemporaryDirectory() as tmpdir:
+        assert is_valid_path_prefix(os.path.join(tmpdir, 'test'))
+
+
+@patch('os.path.exists', return_value=False)
+def test_is_valid_path_prefix_no_directory(mock_exists):
+    assert not is_valid_path_prefix('/nonexistent_dir/test')
+
+
+@patch('os.path.exists', return_value=True)
+def test_is_valid_path_prefix_directory_exists(mock_exists):
+    assert is_valid_path_prefix('/existing_dir/test')
+
+
+if __name__ == "__main__":
+    pytest.main()