[bugfix] fix bug when top_logprobs=0 with spec decoding (#30059)

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

tests/v1/sample/test_logprobs.py

@@ -528,9 +528,11 @@ def test_logprobs_mode(logprobs_mode: LogprobsMode):
         ),
     ],
 )
+@pytest.mark.parametrize("top_logprobs", [0, 3])
 def test_spec_decode_logprobs(
     logprobs_mode: LogprobsMode,
     model_setup: tuple[str, str, str],
+    top_logprobs: int,
 ):
     """Spec decode logprobs should match those of the base model.
 
@@ -543,7 +545,7 @@ def test_spec_decode_logprobs(
 
     prompt = "Hello world " * 50
     sampling_params = SamplingParams(
-        temperature=0, logprobs=3, max_tokens=10, ignore_eos=False
+        temperature=0, logprobs=top_logprobs, max_tokens=10, ignore_eos=False
     )
     method, model_name, spec_model_name = model_setup
     max_model_len = 256

tests/v1/sample/test_rejection_sampler.py

@@ -111,7 +111,7 @@ def create_sampling_metadata(
         top_p=top_p,
         top_k=top_k,
         generators=generators,
-        max_num_logprobs=0,
+        max_num_logprobs=None,
         no_penalties=no_penalties,
         prompt_token_ids=prompt_token_ids,
         frequency_penalties=frequency_penalties,

vllm/v1/sample/rejection_sampler.py

@@ -145,7 +145,7 @@ class RejectionSampler(nn.Module):
         )
 
         logprobs_tensors = None
-        if sampling_metadata.max_num_logprobs:
+        if sampling_metadata.max_num_logprobs is not None:
             logprobs_tensors = self._get_logprobs_tensors(
                 sampling_metadata.max_num_logprobs,
                 metadata,