[xla][gpu] Implement bf16 3way gemm to compute F32 matmul
[xla][gpu] Implement bf16 3way gemm to compute F32 matmul
This CL implements bf16 3way gemm in triton codegen. It uses three BF16 muls to compute one F32 mul instead of 6 as in bf16 6way gemm. The experiments show that its precision is about one order of magnitude less than bf16 6way gemm.
