Disable AVX512 CPU dispatch by default (#80253)
As it can be slower, see https://github.com/pytorch/pytorch/issues/80252
Update trunk test matrix to test AVX512 config in `nogpu_AVX512` flavor.
Kill `nogpu_noAVX` as AVX support were replaced with AVX512 when https://github.com/pytorch/pytorch/pull/61903 were landed
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80253
Approved by: https://github.com/ngimel
