deprecate is_torch_bf16_available
This is a follow up to https://github.com/huggingface/transformers/pull/17734 where @pacman100 discovered that the IPEX PR made `is_torch_bf16_available` ambiguous, as it went from gpu-only checks to cpu or gpu which is undefined behavior. 

So this PR deprecates this function in favor of the very specific `is_torch_bf16_gpu_available` and `is_torch_bf16_cpu_available` that were added in https://github.com/huggingface/transformers/pull/17734

@sgugger 
_The documentation is not available anymore as the PR was closed or merged._<|||||>does it look good now, Sylvain? <|||||>Yes, all good :-) Thanks again!