[Bug]: Option for preemption_mode uses underscore (_) instead of dash (-)

[maharjun]

Your current environment

Irrelevant, the error is easily seen in the code. It is seen in version 0.5.0.post1, in the file vllm.engine.arg_utils:576

🐛 Describe the bug

In the latest release (V0.5.0.post1), the CLI argument for preemption_mode parameter of EngineArgs is --preemption_mode which goes against the typical convention of replacing _ with - in command-line parameters. This won't affect any other code if fixed to --preemption-mode, because argparse converts any dashes to underscores in the namespace returned by parser.parse_args(). However the current situation causes problems when I want to convert an EngineArgs instance to set of command-line flags (say in order to launch a vllm server programmatically via subprocess)

[mgoin]

Thanks for reporting this @maharjun! I've posted a fix here #5688