Remove static initalizers from Vec256 (#38088)
Summary:
Follow up after PR https://github.com/pytorch/pytorch/pull/37767
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38088
Test Plan: `qemu-x86_64 -cpu Broadwell -E ATEN_CPU_CAPABILITY=default ./bin/basic --gtest_filter=BasicTest.BasicTestCPU`
Differential Revision: D21468726
Pulled By: malfet
fbshipit-source-id: dcf9ce3d7816e9bdfe5d0fcf8b7cad42c0f77b4c
