[fix] Clamp logprob with dtype min to prevent -inf (#3224)

python/sglang/srt/layers/sampler.py

@@ -72,9 +72,11 @@ class Sampler(nn.Module):
                     # NOTE: the top_p_renorm_prob from flashinfer has numerical problems,
                     # https://github.com/flashinfer-ai/flashinfer/issues/708
                     # so we use the torch implementation.
+
+                    # clamp to avoid -inf
                     logprobs = torch.log(
                         top_p_normalize_probs_torch(probs, sampling_info.top_ps)
-                    )
+                    ).clamp(min=torch.finfo(probs.dtype).min)
 
                 max_top_k_round, batch_size = 32, probs.shape[0]
                 uniform_samples = torch.rand(
@@ -109,9 +111,10 @@ class Sampler(nn.Module):
                     sampling_info.need_min_p_sampling,
                 )
                 if return_logprob:
+                    # clamp to avoid -inf
                     logprobs = torch.log(
                         top_p_normalize_probs_torch(probs, sampling_info.top_ps)
-                    )
+                    ).clamp(min=torch.finfo(probs.dtype).min)
             else:
                 raise ValueError(
                     f"Invalid sampling backend: {global_server_args_dict['sampling_backend']}"