check sharding + network iso pre optimization

Author: gwang111

src/sagemaker/serve/builder/model_builder.py

@@ -1234,6 +1234,11 @@ def _model_builder_optimize_wrapper(
         Returns:
             Model: A deployable ``Model`` object.
         """
+        if hasattr(self, "enable_network_isolation") and sharding_config:
+            raise ValueError(
+                "EnableNetworkIsolation cannot be set to True since SageMaker Fast Model "
+                "Loading of model requires network access."
+            )
 
         # TODO: ideally these dictionaries need to be sagemaker_core shapes
         # TODO: for organization, abstract all validation behind this fn