[Speculative decoding] Support target-model logprobs (#4378)

tests/spec_decode/utils.py

@@ -201,6 +201,7 @@ def assert_logprobs_dict_allclose(
 def create_sampler_output_list(
         token_ids: torch.Tensor,
         probs: Iterable[Optional[torch.Tensor]],
+        logprobs: Iterable[Optional[torch.Tensor]],
         seq_ids: Optional[List[int]] = None) -> List[SamplerOutput]:
     num_steps, batch_size = token_ids.shape
     token_ids_by_step = token_ids.tolist()
@@ -222,6 +223,7 @@ def create_sampler_output_list(
             ) for seq_index, token_id in enumerate(token_ids_by_step[step])
         ],
                       sampled_token_probs=probs[step],
+                      logprobs=logprobs[step],
                       sampled_token_ids=token_ids[step])
         for step in range(num_steps)
     ]