[Hardware][CPU] Build fix for ARM without BF16

[ericcurtin]

So we can build on older ARM chips like M1 that don't have bf16.

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[gemini-code-assist]

Code Review

This pull request aims to fix a build issue on ARM CPUs that lack BF16 support, like Apple's M1. The change correctly uses a preprocessor directive to conditionally compile a template specialization for c10::BFloat16. I suggest improving the robustness of this preprocessor condition, making it more maintainable for future architecture additions.

[ericcurtin]

PTAL @mgoin @tlrmchlsmth

[ericcurtin]

@simon-mo @dtrifiro PTAL

[ericcurtin]

@bigPYJ1151 should I rebase and hope for the best that the build passed? 😅

[bigPYJ1151]

@bigPYJ1151 should I rebase and hope for the best that the build passed? 😅

@ericcurtin It's not worth to re-run all tests. I'll request for force merge.