[cuBLAS] Add an option to disable reduced precision reductions for BF16 GEMM (#89172)
Essentially the same change as #67946, except that the default is to disallow reduced precision reductions in `BFloat16` GEMMs (for now). If performance is severely regressed, we can change the default, but this option appears to be necessary to pass some `addmm` `BFloat16` tests on H100.
CC @ptrblck @ngimel
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89172
Approved by: https://github.com/ngimel
