Update arg_utils.py with semi_structured_sparse_w16a16

[mgoin]

Tested by checking the help message in openai server:

python -m vllm.entrypoints.openai.api_server --help

Before:

  --sparsity {sparse_w16a16,None}, -s {sparse_w16a16,None}
                        Method used to compress sparse weights. If None, we first check the `sparsity_config`
                        attribute in the model config file. If that is None we assume the model weights are dense

After:

  --sparsity {None,sparse_w16a16,semi_structured_sparse_w16a16}, -s {None,sparse_w16a16,semi_structured_sparse_w16a16}
                        Method used to compress sparse weights. If None, we first check the `sparsity_config`
                        attribute in the model config file. If that is None we assume the model weights are dense