[Bugfix] Add device assertion to TorchSDPA (#5402)

vllm/attention/selector.py

@@ -58,6 +58,9 @@ def get_attn_backend(
             ROCmFlashAttentionBackend)
         return ROCmFlashAttentionBackend
     elif backend == _Backend.TORCH_SDPA:
+        # TODO: make XPU backend available here.
+        assert is_cpu(), RuntimeError(
+            "Torch SDPA backend is only used for the CPU device.")
         logger.info("Using Torch SDPA backend.")
         from vllm.attention.backends.torch_sdpa import TorchSDPABackend
         return TorchSDPABackend