Change launch_environment to pattern 2

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/253

WTTS

Reviewed By: wat3rBro

Differential Revision: D36438058

fbshipit-source-id: 1909cd31481eca92474baf952bd227256c786ac2

d2go/utils/launch_environment.py

 #!/usr/bin/env python3
 # Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
 
+from d2go.utils.oss_helper import fb_overwritable
 
-MODEL_ZOO_STORAGE_PREFIX = "https://mobile-cv.s3-us-west-2.amazonaws.com/d2go/models/"
 
+@fb_overwritable()
+def get_model_zoo_storage_prefix() -> str:
+    return "https://mobile-cv.s3-us-west-2.amazonaws.com/d2go/models/"
 
+
+@fb_overwritable()
 def get_launch_environment():
     return "local"
+
+
+MODEL_ZOO_STORAGE_PREFIX = get_model_zoo_storage_prefix()