Check gpu availability at server args creation

[MrAta]

Motivation

During server args construction we check the gpu memory size to configure knobs like chunked prefill and cuda graph sizes.
For use cases where the args are constructed programmatically on a non-GPU environment and sent to a GPU environment, this fails because we use nvidia-smi to check nv GPU memory size while it's not available in those environments.

Modifications

Add a check for gpu availability before checking the memory size.

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

[ByronHsu]

LGTM to unblock our internal use. cc @merrymercy for the final stamp