fix: Only calculate deploy_key_prefix when needed

The `deploy_key_prefix` variable is only used conditionally in
`Model.prepare_container_def`. It can be avoided when no code upload is
needed.

Author: adamantike

src/sagemaker/model.py

@@ -684,11 +684,11 @@ def prepare_container_def(
         Returns:
             dict: A container definition object usable with the CreateModel API.
         """
-        deploy_key_prefix = fw_utils.model_code_key_prefix(
-            self.key_prefix, self.name, self.image_uri
-        )
         deploy_env = copy.deepcopy(self.env)
         if self.source_dir or self.dependencies or self.entry_point or self.git_config:
+            deploy_key_prefix = fw_utils.model_code_key_prefix(
+                self.key_prefix, self.name, self.image_uri
+            )
             self._upload_code(deploy_key_prefix, repack=self.is_repack())
             deploy_env.update(self._script_mode_env_vars())