use fb. in import path - modeling/backbone/fb

Summary: see bottom diff

Reviewed By: newstzpz

Differential Revision: D31780235

fbshipit-source-id: ec1285c4c5457a631e1eb88bebd47c9f41b47e12

d2go/modeling/backbone/__init__.py

+#!/usr/bin/env python3
+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+
+from . import fbnet_v2
+
+
+# Explicitly expose all registry-based modules
+__all__ = [
+    "fbnet_v2",
+]