add error info to indicate no sliding window for triton backward pass

[scxiao]

Motivation

We tried to run flash attention sliding window case, triton implementation does not support that, but no error info is reported, which generate incorrect results. The sliding window input is ignored.

Technical Details

Added an error message to tell users for that.

Test Plan

There are existing tests

Test Result

passes

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 3158 --add-label <label>

[brunomazzottiamd]

LGTM!

The failures in tests/test_flash_attn_triton_amd.py::test_flash_attn_varlen_output test, that's part of Flash Attention - Triton / MI35X (1 GPU) test job, were addressed in #2695. IMHO it isn't a blocker for merging.

[micmelesse]

Let us wait for #2695 to get merged and then we can merge this

[micmelesse]

@scxiao Can you rebase this pr?

[scxiao]

@scxiao Can you rebase this pr?

Thanks. Done.

[brunomazzottiamd]

@micmelesse, we have a Flash Attention Integration UT failure in this PR (on MI350):

=========================== short test summary info ============================
FAILED tests/test_flash_attn_triton_amd.py::test_flash_attn_varlen_output[0.0-0.17-1024-1024-192-True-False-True-False-mha-dtype0-False]
= 1 failed, 263 passed, 2640 deselected, 6 warnings, 2 rerun in 721.78s (0:12:01) =

It's an assertion error on dk comparison. I find this strange... Wasn't this fixed in #2695? Do you have any thoughts on this? Thanks!

[micmelesse]

@brunomazzottiamd I saw that. I am retrying the job. I am also working on this pr that should address these flaky dropout tests #3289