Mostly sync BlasKernel.cpp with ATen ReducedPrecisionGemvFastPathKernel

The two files were similar, but diverged due to recent changes. Since we have sharing of PyTorch headers, we can keep them mostly the same; differences are some of the namespace stuff and a couple of EXECUTORCH NOTEs.

Differential Revision: [D74702689](https://our.internmc.facebook.com/intern/diff/D74702689/)

[ghstack-poisoned]

Author: swolchok