[oneDNN] Add BFloat16 Specialization Functor for Mean Op
@ezhulenev - the AMD ROCm failure is not related to changes in this PR. Can you please help review this PR? Hi @ezhulenev, Can you please review this PR ? Thank you! @cantonios Can you please review this PR also?  Looks like the tests passed. Let me know if I need to change anything. Thanks! It was originally reverted because it broke a set of internal tests across Google back in August.  But those logs are now purged, so I can't see why exactly.

We can try again to see if it triggers the same errors.
Resubmitting changes in PR #61331. Not sure why they were reverted.

Currently BFloat16 Mean Op causes BFloat16 accumulation which may result in incorrect output. This prevents Mean to be used with the lower BFloat16 precision. This PR:

Just like the existing implementation of the Sum op, ensures BFloat16 Mean accumulation happens in FP32 by adding a Casting Specialization registration.
Adds Benchmark and Kernel tests to verify the implementation.
Adds Mean back to the Infer List from Deny List and fixes a typo in the Infer List initialization.
