use accumulate type in BF16 gemm(include dot, mv) ref path (#96074)
Fix https://github.com/pytorch/pytorch/issues/95125 and https://github.com/pytorch/pytorch/issues/83863 for bf16 accumulation in gemm ref path
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96074
Approved by: https://github.com/lezcano, https://github.com/peterbell10
