[platforms] enable platform plugins (#11602)

Signed-off-by: youkaichao <youkaichao@gmail.com>

tests/plugins_tests/test_platform_plugins.py

@@ -0,0 +1,16 @@
+def test_platform_plugins():
+    # simulate workload by running an example
+    import runpy
+    current_file = __file__
+    import os
+    example_file = os.path.join(
+        os.path.dirname(os.path.dirname(os.path.dirname(current_file))),
+        "examples", "offline_inference.py")
+    runpy.run_path(example_file)
+
+    # check if the plugin is loaded correctly
+    from vllm.platforms import _init_trace, current_platform
+    assert current_platform.device_name == "DummyDevice", (
+        f"Expected DummyDevice, got {current_platform.device_name}, "
+        "possibly because current_platform is imported before the plugin"
+        f" is loaded. The first import:\n{_init_trace}")