[XPU] Delay BF16 check to worker init for spawn compatibility (#22979)

Signed-off-by: chzhang <chaojun.zhang@intel.com>

vllm/platforms/rocm.py

@@ -462,3 +462,23 @@ class RocmPlatform(Platform):
     def is_kv_cache_dtype_supported(cls, kv_cache_dtype: str,
                                     model_config: "ModelConfig") -> bool:
         return True
+
+    @classmethod
+    def check_if_supports_dtype(cls, torch_dtype: torch.dtype):
+        if torch_dtype == torch.bfloat16:  # noqa: SIM102
+            if not cls.has_device_capability(80):
+                capability = cls.get_device_capability()
+                gpu_name = cls.get_device_name()
+
+                if capability is None:
+                    compute_str = "does not have a compute capability"
+                else:
+                    version_str = capability.as_version_str()
+                    compute_str = f"has compute capability {version_str}"
+
+                raise ValueError(
+                    "Bfloat16 is only supported on GPUs "
+                    "with compute capability of at least 8.0. "
+                    f"Your {gpu_name} GPU {compute_str}. "
+                    "You can use float16 instead by explicitly setting the "
+                    "`dtype` flag in CLI, for example: --dtype=half.")