Mark subparser argument as optional

By parking the subparser as required, running just `vllm` produces an error.

```
$ vllm
usage: vllm [-h] [-v] {chat,complete,serve} ...
vllm: error: the following arguments are required: subparser
```

With this fix, running `vllm` will just show the help message as if you rbryant
`vllm --help` or `vllm -h`.

Signed-off-by: Russell Bryant <[email protected]>

Author: russellb

vllm/cmd/main.py

@@ -57,7 +57,7 @@ def main():
                         '--version',
                         action='version',
                         version=vllm.version.__version__)
-    subparsers = parser.add_subparsers(required=True, dest="subparser")
+    subparsers = parser.add_subparsers(required=False, dest="subparser")
     cmds = {}
     for cmd_module in CMD_MODULES:
         new_cmds = cmd_module.cmd_init()