JIT Batch Norm fusion (#15897)
Summary:
Resubmit of #15146, which has been accidentally reverted.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15897
Differential Revision: D13616093
Pulled By: zou3519
fbshipit-source-id: 0c3a3bec8f9fed57274da9f6c7cf40cbc05cf91a
