[k8s] Use default runtime on k3s for CPU tasks (#3918)

* fix

* lint

sky/provision/kubernetes/instance.py

@@ -530,7 +530,9 @@ def _create_pods(region: str, cluster_name_on_cloud: str,
                        'override runtimeClassName in ~/.sky/config.yaml. '
                        'For more details, refer to https://skypilot.readthedocs.io/en/latest/reference/config.html')  # pylint: disable=line-too-long
 
-    if nvidia_runtime_exists:
+    needs_gpus = (pod_spec['spec']['containers'][0].get('resources', {}).get(
+        'limits', {}).get('nvidia.com/gpu', 0) > 0)
+    if nvidia_runtime_exists and needs_gpus:
         pod_spec['spec']['runtimeClassName'] = 'nvidia'
 
     created_pods = {}