[bug] fix WeightTransferConfig.backend to allow for all strings (#43121)

Signed-off-by: ahao-anyscale <ahao@anyscale.com>

tests/distributed/test_weight_transfer.py

@@ -173,20 +173,7 @@ class TestEngineRegistry:
 
     def test_create_engine_invalid_backend(self):
         """Test factory raises for invalid backend."""
-        # Pydantic validates Literal types at construction, so we can't create
-        # a config with an invalid backend. Instead, we test by directly
-        # accessing the registry or using model_construct to bypass validation.
-        from pydantic import ValidationError
-
-        # Test that Pydantic prevents invalid backend at construction
-        with pytest.raises(ValidationError):
-            WeightTransferConfig(backend="invalid")
-
-        # Test factory error by creating a config with valid backend but
-        # then manually modifying the backend attribute (bypassing validation)
-        config = WeightTransferConfig(backend="nccl")
-        # Use object.__setattr__ to bypass Pydantic validation
-        object.__setattr__(config, "backend", "invalid")
+        config = WeightTransferConfig(backend="invalid")
         parallel_config = create_mock_parallel_config()
         with pytest.raises(ValueError, match="Invalid weight transfer backend"):
             WeightTransferEngineFactory.create_engine(config, parallel_config)

vllm/config/weight_transfer.py

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
-from typing import Literal
-
 from vllm.config.utils import config
 
 
@@ -9,5 +7,7 @@ from vllm.config.utils import config
 class WeightTransferConfig:
     """Configuration for weight transfer during RL training."""
 
-    backend: Literal["nccl", "ipc"] = "nccl"
-    """The backend to use for weight transfer."""
+    backend: str = "nccl"
+    """The backend to use for weight transfer. Validated against the
+    `WeightTransferEngineFactory` registry at engine creation time.
+    """