[Bugfix] Check chain_speculative_sampling before calling it (vllm-project#11673)

Signed-off-by: Lu Fang <[email protected]>

Author: houseroad

vllm/model_executor/layers/rejection_sampler.py

@@ -118,7 +118,7 @@ def forward(
 
         # If use Flashinfer chain_speculative_sampling kernel
         # for rejection sampling
-        if self.use_flashinfer:
+        if self.use_flashinfer and chain_speculative_sampling is not None:
             batch_size, k, _ = draft_probs.shape
             uniform_samples = self._create_uniform_samples(
                 seeded_seqs, batch_size, k, draft_probs.device)