[Bugfix] Fix vllm bench ... on CPU-only head nodes (#25283)

Signed-off-by: Aydin Abiar <aydin@anyscale.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Aydin Abiar <aydin@anyscale.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>

vllm/entrypoints/cli/main.py

@@ -8,6 +8,11 @@ to avoid certain eager import breakage."""
 from __future__ import annotations
 
 import importlib.metadata
+import sys
+
+from vllm.logger import init_logger
+
+logger = init_logger(__name__)
 
 
 def main():
@@ -29,6 +34,22 @@ def main():
 
     cli_env_setup()
 
+    # For 'vllm bench *': use CPU instead of UnspecifiedPlatform by default
+    if len(sys.argv) > 1 and sys.argv[1] == "bench":
+        logger.debug(
+            "Bench command detected, must ensure current platform is not "
+            "UnspecifiedPlatform to avoid device type inference error"
+        )
+        from vllm import platforms
+
+        if platforms.current_platform.is_unspecified():
+            from vllm.platforms.cpu import CpuPlatform
+
+            platforms.current_platform = CpuPlatform()
+            logger.info(
+                "Unspecified platform detected, switching to CPU Platform instead."
+            )
+
     parser = FlexibleArgumentParser(
         description="vLLM CLI",
         epilog=VLLM_SUBCMD_PARSER_EPILOG.format(subcmd="[subcommand]"),