add flatbuffer_loader and flatbuffer_serializer as BUCK target (#71463)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71463
title
Test Plan: unittest
Reviewed By: zhxchen17
Differential Revision: D33651339
fbshipit-source-id: 4bf325a40e263a441fd86bce560645ad0c1ebb23
(cherry picked from commit 4cb02e62a68f338e3388ad09276ced9b8f4cdcb1)
