Map top-left map to bottom-right for self-attn

[Micky774]

Motivation

gfx1250 only has kernels which support bottom-right masking. In the case of s_q=s_kv we have that bottom-right = top-left masking. This mirrors a similar symmetry already in place for gfx942.

Technical Details

Test Plan

Tested by building w/ TE and applying to E2E models.

Test Result

The E2E model succeeds.

Submission Checklist

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

[github-actions]

🏷️ CI Guide

Runs automatically on every PR:

• ✅ Pre-checks (submodule verification, code formatting)
• ✅ Aiter op tests (gfx942 + gfx950)
• ✅ Triton tests on MI35X (only when aiter/ops/triton/** or related paths are changed)

Extended tests (opt-in via labels):

Label	Tests
ci:triton-300x	Run an additional Triton test job on MI300X in PRs; main branch always runs both MI35X and MI300X
ci:sglang	SGLang integration tests: DeepSeek-R1-MXFP4 accuracy, Qwen 3.5 accuracy
ci:atom	ATOM benchmark: DeepSeek-R1-0528, GPT-OSS-120B
ci:atom_full	ATOM accuracy suite for PR and main models from ATOM models_accuracy.json
ci:vllm	vLLM benchmark: GPT-OSS-120B, DeepSeek-R1-0528, Kimi-K2.5
ci:all	All standard extended tests (excludes ci:atom_full)

Only add ci:atom_full for FlyDSL or Triton upgrades.
Add labels via the sidebar or gh pr edit 3939 --add-label <label>

[Copilot]

Pull request overview

Updates the FMHA v3 backward ASM kernel selection to handle gfx1250’s limited causal-mask kernel availability by remapping an equivalent causal mask in the self-attention (square) case, aligning with an existing symmetry already used for gfx942.

Changes:

• For arch_id == "gfx1250", remap mask_type from top-left (1) to bottom-right (2) when seqlen_q == seqlen_k so existing gfx1250 causal BWD kernels can be selected.
• Keep the existing gfx942 square-seqlen remap logic (2 → 1) and apply the analogous inverse remap for gfx1250.

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