[Model Runner V2] Add probabilistic rejection sampling for spec decoding (#35461)

Signed-off-by: Giancarlo Delfin <gdelfin@inferact.ai>

vllm/config/speculative.py

@@ -57,6 +57,10 @@ SpeculativeMethod = Literal[
     EagleModelTypes,
     NgramGPUTypes,
 ]
+RejectionSampleMethod = Literal[
+    "strict",
+    "probabilistic",
+]
 
 
 @config
@@ -171,6 +175,12 @@ class SpeculativeConfig:
     """Load config for the draft model. If not specified, will use the load
     config from the target model."""
 
+    rejection_sample_method: RejectionSampleMethod = "strict"
+    """Whether to use strict (target and draft sampled tokens match exactly)
+    or probabilistic rejection sampling. Both respect the target model
+    distribution, but the latter yields a higher acceptance rate at the cost
+    of more memory to cache draft logits."""
+
     def compute_hash(self) -> str:
         """
         WARNING: Whenever a new field is added to this config,