[V1] Enable multi-input by default (#15799)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/config.py

@@ -2667,14 +2667,20 @@ class MultiModalConfig:
                                usedforsecurity=False).hexdigest()
         return hash_str
 
+    def get_default_limit_per_prompt(self) -> int:
+        """
+        Return the default number of input items allowed per prompt
+        for any modality if not specified by the user.
+        """
+        return 999 if envs.VLLM_USE_V1 else 1
+
     def get_limit_per_prompt(self, modality: str) -> int:
         """
         Get the maximum number of input items allowed per prompt
         for the given modality.
-
-        If not set by the user, this defaults to `1`.
         """
-        return self.limit_per_prompt.get(modality, 1)
+        default = self.get_default_limit_per_prompt()
+        return self.limit_per_prompt.get(modality, default)
 
     # TODO: Add configs to init vision tower or not.