Upgrade MKL-DNN to v0.21.1 (#27597)
Summary:
1. Upgrade MKL-DNN to v0.21.1
2. Fix runtime error on legacy hardware with gcc8
3. Remove workaround for issue https://github.com/pytorch/pytorch/issues/21597
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27597
Differential Revision: D17891492
Pulled By: bddppq
fbshipit-source-id: ab390a655f7ab7fb7144e2c333f25af85a0f5183
