Fix FP8 Rowwise Gemm Compilation with Auto-functionalize V2

Summary:
X-link: facebookresearch/FBGEMM#541

Torch recently introduced auto_functionalized_v2, which makes custom functions pickier about how they are defined. Specifically, torch no longer allows optional preallocated outputs. A custom function must either allocate a tensor and return it, or directly write to a preallocated output and return nothing.

This conflicts with our impelemtnation of f8f8bf16_rowwise and could cause confusing behaviors or errors when compiled. The only solution is to split into two functions with correct signatures. This diff adds `f8f8bf16_rowwise_out`, which is a very thin wrapper that allows preallocated outputs. It's a bit annoying, but this should allow both versions of the function to compile correctly.

Differential Revision: D66795225

Author: jwfromm