Change top_k to be disabled with 0 (still accept -1 for now) (#17773)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/sampling_params.py

@@ -149,7 +149,7 @@ class SamplingParams(
         top_p: Float that controls the cumulative probability of the top tokens
             to consider. Must be in (0, 1]. Set to 1 to consider all tokens.
         top_k: Integer that controls the number of top tokens to consider. Set
-            to -1 to consider all tokens.
+            to 0 (or -1) to consider all tokens.
         min_p: Float that represents the minimum probability for a token to be
             considered, relative to the probability of the most likely token.
             Must be in [0, 1]. Set to 0 to disable this.
@@ -209,7 +209,7 @@ class SamplingParams(
     repetition_penalty: float = 1.0
     temperature: float = 1.0
     top_p: float = 1.0
-    top_k: int = -1
+    top_k: int = 0
     min_p: float = 0.0
     seed: Optional[int] = None
     stop: Optional[Union[str, list[str]]] = None
@@ -256,7 +256,7 @@ class SamplingParams(
         repetition_penalty: Optional[float] = 1.0,
         temperature: Optional[float] = 1.0,
         top_p: Optional[float] = 1.0,
-        top_k: int = -1,
+        top_k: int = 0,
         min_p: float = 0.0,
         seed: Optional[int] = None,
         stop: Optional[Union[str, list[str]]] = None,
@@ -376,7 +376,7 @@ class SamplingParams(
         if self.temperature < _SAMPLING_EPS:
             # Zero temperature means greedy sampling.
             self.top_p = 1.0
-            self.top_k = -1
+            self.top_k = 0
             self.min_p = 0.0
             self._verify_greedy_sampling()
 
@@ -404,8 +404,9 @@ class SamplingParams(
                 f"temperature must be non-negative, got {self.temperature}.")
         if not 0.0 < self.top_p <= 1.0:
             raise ValueError(f"top_p must be in (0, 1], got {self.top_p}.")
-        if self.top_k < -1 or self.top_k == 0:
-            raise ValueError(f"top_k must be -1 (disable), or at least 1, "
+        # quietly accept -1 as disabled, but prefer 0
+        if self.top_k < -1:
+            raise ValueError(f"top_k must be 0 (disable), or at least 1, "
                              f"got {self.top_k}.")
         if not isinstance(self.top_k, int):
             raise TypeError(