[rocm-libraries] ROCm/rocm-libraries#8350 (commit f92ded1)

Add tile shape for FMHA batch prefill on MI308X (on fp8,
 hdim=256) (#8350)

## Motivation

Add a tile size appropriate for FMHA batch prefill fp8/hdim256 on MI308X

## Technical Details

Appending the tile shape to the existing factory such that it can be
picked up by Aiter

## Test Plan

Ran the performance test on both MI300X and MI308X

## Test Result

MI300X performance seems unaffected by this change. MI308X does improve.

## Submission Checklist

- [X] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

Co-authored-by: Damien Lejeune <damien.lejeune@amd.com>