Guard FlashInfer sampler using the same check as FlashInfer attention backend (#29415)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

vllm/v1/sample/ops/topk_topp_sampler.py

@@ -33,6 +33,16 @@ class TopKTopPSampler(nn.Module):
             and current_platform.is_cuda()
         ):
             if envs.VLLM_USE_FLASHINFER_SAMPLER:
+                from vllm.v1.attention.backends.flashinfer import FlashInferBackend
+
+                capability = current_platform.get_device_capability()
+                assert capability is not None
+                if not FlashInferBackend.supports_compute_capability(capability):
+                    capability_str = capability.as_version_str()
+                    raise RuntimeError(
+                        "FlashInfer does not support compute capability "
+                        f"{capability_str}, unset VLLM_USE_FLASHINFER_SAMPLER=1."
+                    )
                 # Users must opt in explicitly via VLLM_USE_FLASHINFER_SAMPLER=1.
                 logger.info_once(
                     "Using FlashInfer for top-p & top-k sampling.",