Make ForcedUnroll usage in bf16 BlasKernel actually work for -Oz builds (#5247)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5247

Clang is very resistant to inlining under -Oz. For ForcedUnroll to actually unroll, we need to force-inline the lambda.
ghstack-source-id: 242067820
exported-using-ghexport

Reviewed By: kimishpatel

Differential Revision: D62154247

fbshipit-source-id: bcf730f16e4128073fefe6160b5c728cafc1cb98