Move DistributonTemplates to anonymous namespace (#37429)
Summary:
All templates which are included from `ATen/native/cpu` must be in anonymous namespace, especially if they are using instruction set extensions but do not support dynamic dispatching.
Otherwise, linker is free to pick AVX2, AVX or DEFAULT version of instantiated templates during final linking stage
Test Plan; Apply on top of https://github.com/pytorch/pytorch/pull/37121 and make sure that `basic` test successfully finishes on CircleCI MacPro (that does not support AVX2), but `ATEN_CPU_CAPABILITY=avx2 ./basic --gtest_filter=*HalfCPU` crashes with illegal instruction
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37429
Differential Revision: D21294818
Pulled By: malfet
fbshipit-source-id: ab32b8553de225d2f672fac2f48591682bd7dec4
