Automatically tell users that dict args must be valid JSON in CLI (#17577)

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

vllm/engine/arg_utils.py

@@ -150,7 +150,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
 
         # Get the help text for the field
         name = field.name
-        help = cls_docs[name]
+        help = cls_docs[name].strip()
         # Escape % for argparse
         help = help.replace("%", "%%")
 
@@ -165,6 +165,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
             type_hints.add(field.type)
 
         # Set other kwargs based on the type hints
+        json_tip = "\n\nShould be a valid JSON string."
         if contains_type(type_hints, bool):
             # Creates --no-<name> and --<name> flags
             kwargs[name]["action"] = argparse.BooleanOptionalAction
@@ -201,6 +202,7 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
         elif contains_type(type_hints, dict):
             # Dict arguments will always be optional
             kwargs[name]["type"] = optional_type(json.loads)
+            kwargs[name]["help"] += json_tip
         elif (contains_type(type_hints, str)
               or any(is_not_builtin(th) for th in type_hints)):
             kwargs[name]["type"] = str