Fix empty output when temp is too low (#2937)

Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

vllm/model_executor/layers/sampler.py

@@ -118,8 +118,9 @@ class Sampler(nn.Module):
                                       sampling_tensors.frequency_penalties,
                                       sampling_tensors.repetition_penalties)
 
-        # Apply temperature scaling.
+        # Use float32 to apply temperature scaling.
         # Use in-place division to avoid creating a new tensor.
+        logits = logits.to(torch.float)
         logits.div_(sampling_tensors.temperatures.unsqueeze(dim=1))
 
         if do_top_p_top_k:

vllm/sampling_params.py

@@ -13,6 +13,7 @@ from vllm.logger import init_logger
 logger = init_logger(__name__)
 
 _SAMPLING_EPS = 1e-5
+_MAX_TEMP = 1e-2
 
 
 class SamplingType(IntEnum):
@@ -145,6 +146,12 @@ class SamplingParams:
         self.presence_penalty = presence_penalty
         self.frequency_penalty = frequency_penalty
         self.repetition_penalty = repetition_penalty
+        if 0 < temperature < _MAX_TEMP:
+            logger.warning(
+                "temperature %s is less than %s, which may cause numerical "
+                "errors nan or inf in tensors. We have maxed it out to %s.",
+                temperature, _MAX_TEMP, _MAX_TEMP)
+            temperature = max(temperature, _MAX_TEMP)
         self.temperature = temperature
         self.top_p = top_p
         self.top_k = top_k