Added Mean and Variance calculation function. (#39986)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39986
Mean and Variance computation to match with Intel NNPI implementation.
Test Plan: Manual Testing
Reviewed By: hyuen
Differential Revision: D22008566
fbshipit-source-id: 6ac4563859b84121a2482f8e2f738be5c6111f57
