[Misc] vllm CLI flags should be ordered for better user readability (#10017)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

vllm/utils.py

@@ -1148,9 +1148,23 @@ class StoreBoolean(argparse.Action):
                              "Expected 'true' or 'false'.")
 
 
+class SortedHelpFormatter(argparse.HelpFormatter):
+    """SortedHelpFormatter that sorts arguments by their option strings."""
+
+    def add_arguments(self, actions):
+        actions = sorted(actions, key=lambda x: x.option_strings)
+        super(SortedHelpFormatter, self).add_arguments(actions)
+
+
 class FlexibleArgumentParser(argparse.ArgumentParser):
     """ArgumentParser that allows both underscore and dash in names."""
 
+    def __init__(self, *args, **kwargs):
+        # Set the default 'formatter_class' to SortedHelpFormatter
+        if 'formatter_class' not in kwargs:
+            kwargs['formatter_class'] = SortedHelpFormatter
+        super().__init__(*args, **kwargs)
+
     def parse_args(self, args=None, namespace=None):
         if args is None:
             args = sys.argv[1:]