Make sure temporary vectors are properly initialized in avx2 code (#32722)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32722
Checked using [this](https://godbolt.org/z/uAaE9R) that it gives the correct assembly.
Test Plan: Imported from OSS
Differential Revision: D19610012
Pulled By: albanD
fbshipit-source-id: 4d1cb812951ae03d412a0fba3c80730f0d286e1f
