add cpu option for p/d in nixl_connector (#28356)

Signed-off-by: Hongming Zheng <hongming.zheng@intel.com>

vllm/distributed/kv_transfer/kv_connector/v1/nixl_connector.py

@@ -91,6 +91,7 @@ _NIXL_SUPPORTED_DEVICE = {
     ),
     "tpu": ("cpu",),
     "xpu": ("cpu",),
+    "cpu": ("cpu",),
 }
 # support for oot platform by providing mapping in current_platform
 _NIXL_SUPPORTED_DEVICE.update(current_platform.get_nixl_supported_devices())
@@ -348,7 +349,13 @@ class NixlConnectorScheduler:
             + vllm_config.parallel_config.data_parallel_rank
         )
         assert vllm_config.kv_transfer_config is not None
-        self.use_host_buffer = vllm_config.kv_transfer_config.kv_buffer_device == "cpu"
+        if current_platform.device_type == "cpu":
+            self.use_host_buffer = False
+        else:
+            self.use_host_buffer = (
+                vllm_config.kv_transfer_config.kv_buffer_device == "cpu"
+            )
+
         logger.info("Initializing NIXL Scheduler %s", engine_id)
 
         # Background thread for handling new handshake requests.
@@ -820,7 +827,11 @@ class NixlConnectorWorker:
         # cpu kv buffer for xfer
         # used when device memory can not be registered under nixl
         self.host_xfer_buffers: dict[str, torch.Tensor] = {}
+        if self.device_type == "cpu":
+            self.use_host_buffer = False
+        else:
             self.use_host_buffer = self.kv_buffer_device == "cpu"
+
         # support for oot platform which can't register nixl memory
         # type based on kv_buffer_device
         nixl_memory_type = current_platform.get_nixl_memory_type()
@@ -1021,6 +1032,9 @@ class NixlConnectorWorker:
         # Set a no-op if the host buffer is not cpu.
         if self.kv_buffer_device != "cpu":
             return
+        # Set a no-op if self.device_type is 'cpu'.
+        if self.device_type == "cpu":
+            return
         assert self.use_host_buffer
         self.copy_blocks = copy_operation