remove redundant contiguous in unfold_backward. (#38871)
Summary:
As per title. Makes for a 5-25% boost on CPU in tests from [https://github.com/pytorch/pytorch/issues/36612](https://github.com/pytorch/pytorch/pull/36612).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38871
Differential Revision: D21687007
Pulled By: albanD
fbshipit-source-id: c64b2545ad159fa7463cc32f2e2a72dde6229eff
