[ROCm] Use supports_fp8() for FP8 feature gates instead of arch checks

[laudney]

Summary

Replace verbose architecture-specific FP8 capability checks (on_gfx9(), on_mi3xx(), has_device_capability(94)) with the cross-platform current_platform.supports_fp8() predicate across all FP8-related code paths.

This is a small refactoring PR that unblocks FP8 features on RDNA4 (gfx12) GPUs which support FP8 but were excluded by MI300-specific gates. The supports_fp8() method already correctly covers MI300, gfx950, gfx12 on ROCm and capability >= 8.9 on CUDA — this PR simply switches the callers to use it.

Changes (5 files, net -27 lines)

File	Before	After
fused_batched_moe.py	10-line on_gfx9() + CUDA capability check	supports_fp8() one-liner
fused_moe.py	Same 10-line pattern	supports_fp8() one-liner
scaled_mm/rocm.py	on_mi3xx() — excluded gfx12	supports_fp8() — includes gfx12
scaled_mm/pytorch.py	on_mi3xx() + capability >= 94	supports_fp8()
ptpc_fp8.py	has_device_capability(94)	supports_fp8() + updated docstring

Why this matters

Without this change, RDNA4 GPUs fall through to non-FP8 paths even though they have hardware FP8 support (v_dot4_f32_fp8_fp8, FP8 format in torch._scaled_mm). The old on_mi3xx()/on_gfx9() gates were written before RDNA4 existed.

Related PRs (RDNA4/gfx12 series)

• [ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode #34709 — [ROCm] Enable wvSplitK/wvSplitKQ skinny GEMM kernels for RDNA4
• [ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12 #34632 — [ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12
• [ROCm] Make Whisper causal attention backend-agnostic #34631 — [ROCm] Make Whisper causal attention backend-agnostic

Test plan

• Qwen3-14B-FP8 (block-wise FP8) serving on gfx1201 — FP8 rowwise matmul path now activates
• Qwen3-Coder-30B-A3B AWQ-4bit — FP8 MoE gate correctly enables Triton FP8 experts
• No regression on MI300-series (predicate returns same result as old checks)
• No regression on CUDA (predicate delegates to has_device_capability(8, 9))

[gemini-code-assist]

Code Review

This pull request is a well-executed refactoring that replaces verbose, architecture-specific FP8 capability checks with a unified current_platform.supports_fp8() predicate. The changes are applied consistently across multiple files, simplifying the code and improving maintainability. Most importantly, this change correctly enables FP8 features on newer RDNA4 (gfx12) GPUs, which were previously excluded by MI300-specific gates. The updated error messages and docstrings are also clearer and more generic. The changes are correct and a clear improvement to the codebase.

[mergify]

Hi @laudney, the pre-commit checks have failed. Please run:

uv pip install pre-commit
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy or markdownlint failing?

mypy and markdownlint are run differently in CI. If the failure is related to either of these checks, please use the following commands to run them locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10
# For markdownlint
pre-commit run --hook-stage manual markdownlint

[yewentao256]

LGTM, thanks for the work!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @laudney.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[laudney]

Hey, this is approved and rebased on latest main. What else do I need to do to get it merged?

[wendyliu235]

@laudney We added a new Intel CI pipeline that only gates Intel PRs, so it does not apply to your PR. Feel free to ignore the result.

[laudney]

Rebased onto latest main. The only conflict was with ptpc_fp8.py which was removed upstream in #32700 — accepted the deletion since the file no longer exists. All other changes applied cleanly. Pre-commit hooks pass (the mypy failures are pre-existing on main, unrelated to this PR).