fix: don't use os.path.join for S3 path when repacking TFS model (aws#1302)

S3 paths always use forward slashes ("/") regardless of the OS,
so using an os.path.join will cause issues when the user is on Windows.

Author: laurenyu

src/sagemaker/tensorflow/serving.py

@@ -14,7 +14,6 @@
 from __future__ import absolute_import
 
 import logging
-import os
 
 import sagemaker
 from sagemaker.content_types import CONTENT_TYPE_JSON
@@ -229,7 +228,7 @@ def prepare_container_def(self, instance_type, accelerator_type=None):
             key_prefix = sagemaker.fw_utils.model_code_key_prefix(self.key_prefix, self.name, image)
 
             bucket = self.bucket or self.sagemaker_session.default_bucket()
-            model_data = "s3://" + os.path.join(bucket, key_prefix, "model.tar.gz")
+            model_data = "s3://{}/{}/model.tar.gz".format(bucket, key_prefix)
 
             sagemaker.utils.repack_model(
                 self.entry_point,