[Misc] Set default kv_buffer_device in a better way (#36862)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/kv_transfer.py

@@ -13,6 +13,12 @@ KVConsumer = Literal["kv_consumer", "kv_both"]
 KVRole = Literal[KVProducer, KVConsumer]
 
 
+def kv_buffer_device_default_factory() -> str:
+    from vllm.platforms import current_platform
+
+    return current_platform.device_type
+
+
 @config
 class KVTransferConfig:
     """Configuration for distributed KV cache transfer."""
@@ -24,9 +30,9 @@ class KVTransferConfig:
     engine_id: str | None = None
     """The engine id for KV transfers."""
 
-    kv_buffer_device: str | None = None
+    kv_buffer_device: str = field(default_factory=kv_buffer_device_default_factory)
     """The device used by kv connector to buffer the KV cache. Choices are
-    'cuda','cpu' and 'xpu'."""
+    'cuda', 'cpu' and 'xpu'."""
 
     kv_buffer_size: float = 1e9
     """The buffer size for TorchDistributedConnector. Measured in number of
@@ -100,11 +106,6 @@ class KVTransferConfig:
                 f"is set, supported roles are {get_args(KVRole)}"
             )
 
-        if self.kv_buffer_device is None:
-            from vllm.platforms import current_platform
-
-            self.kv_buffer_device = current_platform.device_type
-
     @property
     def is_kv_transfer_instance(self) -> bool:
         return self.kv_connector is not None and self.kv_role in get_args(KVRole)