[Bugfix] Fix spec decoding when seed is none in a batch (#10863)

Signed-off-by: Wallas Santos <wallashss@ibm.com>

vllm/model_executor/layers/rejection_sampler.py

@@ -1,6 +1,6 @@
 from functools import cached_property
 from importlib.util import find_spec
-from typing import Dict, List, Optional, Tuple
+from typing import Dict, Optional, Tuple
 
 import torch
 import torch.jit
@@ -386,16 +386,12 @@ def _multinomial(
     if not seeded_seqs:
         q.exponential_(1.0)
     else:
-        non_seeded_indices: List[int] = []
         start = 0
         for idx in range(len(q) // k):
             end = start + k
             generator = seeded_seqs.get(idx)
-            if generator is None:
-                non_seeded_indices.extend(list(range(start, end)))
-            else:
-                q[start:end].exponential_(1.0, generator=generator)
+            # Note: generator might be None for non seeded
+            q[start:end].exponential_(1.0, generator=generator)
             start = end
-        q[non_seeded_indices].exponential_(1.0)
 
     return probs.div_(q).argmax(dim=1).view(-1, num_samples)