[Core] Support model loader plugins (#21067)

Signed-off-by: 22quinn <33176974+22quinn@users.noreply.github.com>

tests/model_executor/model_loader/test_registry.py

@@ -0,0 +1,37 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+from torch import nn
+
+from vllm.config import LoadConfig, ModelConfig
+from vllm.model_executor.model_loader import (get_model_loader,
+                                              register_model_loader)
+from vllm.model_executor.model_loader.base_loader import BaseModelLoader
+
+
+@register_model_loader("custom_load_format")
+class CustomModelLoader(BaseModelLoader):
+
+    def __init__(self, load_config: LoadConfig) -> None:
+        super().__init__(load_config)
+
+    def download_model(self, model_config: ModelConfig) -> None:
+        pass
+
+    def load_weights(self, model: nn.Module,
+                     model_config: ModelConfig) -> None:
+        pass
+
+
+def test_register_model_loader():
+    load_config = LoadConfig(load_format="custom_load_format")
+    assert isinstance(get_model_loader(load_config), CustomModelLoader)
+
+
+def test_invalid_model_loader():
+    with pytest.raises(ValueError):
+
+        @register_model_loader("invalid_load_format")
+        class InValidModelLoader:
+            pass