[Platform] Do not raise error if _Backend is not found (#12023)

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>
Signed-off-by: Mengqing Cao <cmq0113@163.com>
Co-authored-by: Mengqing Cao <cmq0113@163.com>

tests/plugins_tests/test_platform_plugins.py

@@ -1,3 +1,10 @@
+import torch
+
+from tests.kernels.utils import override_backend_env_variable
+from vllm.attention.selector import get_attn_backend
+from vllm.utils import STR_INVALID_VAL
+
+
 def test_platform_plugins():
     # simulate workload by running an example
     import runpy
@@ -14,3 +21,10 @@ def test_platform_plugins():
         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}")
+
+
+def test_oot_attention_backend(monkeypatch):
+    # ignore the backend env variable if it is set
+    override_backend_env_variable(monkeypatch, STR_INVALID_VAL)
+    backend = get_attn_backend(16, torch.float16, torch.float16, 16, False)
+    assert backend.get_name() == "Dummy_Backend"