[V1][Spec Decode] Use better defaults for N-gram (#15358)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/config.py

@@ -2012,18 +2012,30 @@ class SpeculativeConfig:
         if self.method in ("ngram", "[ngram]"):
             # Unified to "ngram" internally
             self.method = "ngram"
-            if self.prompt_lookup_min is None:
-                self.prompt_lookup_min = 1
-            if self.prompt_lookup_max is None or self.prompt_lookup_max < 1:
-                raise ValueError("prompt_lookup_max="
-                                 f"{self.prompt_lookup_max} must be > 0")
+            # Set default values if not provided
+            if (self.prompt_lookup_min is None
+                    and self.prompt_lookup_max is None):
+                # TODO(woosuk): Tune these values. They are arbitrarily chosen.
+                self.prompt_lookup_min = 5
+                self.prompt_lookup_max = 5
+            elif self.prompt_lookup_min is None:
+                assert self.prompt_lookup_max is not None
+                self.prompt_lookup_min = self.prompt_lookup_max
+            elif self.prompt_lookup_max is None:
+                assert self.prompt_lookup_min is not None
+                self.prompt_lookup_max = self.prompt_lookup_min
+
+            # Validate values
             if self.prompt_lookup_min < 1:
-                raise ValueError("prompt_lookup_min="
-                                 f"{self.prompt_lookup_min} must be > 0")
+                raise ValueError(
+                    f"prompt_lookup_min={self.prompt_lookup_min} must be > 0")
+            if self.prompt_lookup_max < 1:
+                raise ValueError(
+                    f"prompt_lookup_max={self.prompt_lookup_max} must be > 0")
             if self.prompt_lookup_min > self.prompt_lookup_max:
-                raise ValueError(f"prompt_lookup_min={self.prompt_lookup_min} "
-                                 "cannot be larger than prompt_lookup_max="
-                                 f"{self.prompt_lookup_max}")
+                raise ValueError(
+                    f"prompt_lookup_min={self.prompt_lookup_min} must "
+                    f"be <= prompt_lookup_max={self.prompt_lookup_max}")
 
             # TODO: current we still need extract vocab_size from target model
             # config, in future, we may try refactor it out, and set