feat(hip,aiter): default silu_and_mul and rope auto backend to native

[demandal25]

Summary

Make backend="auto" resolve to the in-tree HIP native kernel (instead of routing to AITER on gfx942/gfx950) for silu_and_mul and the cos/sin-cache rope ops. Experimentation found the native kernels to be the better default for these two ops; the C++ AITER path stays reachable via an explicit backend="aiter".

What changed

• flashinfer/activation.py — _auto_select_silu_and_mul_backend now always returns "native"; the silu_and_mul docstring describes the new auto behavior.
• flashinfer/rope.py — _auto_select_rope_backend now always returns "native"; both apply_rope_with_cos_sin_cache and its inplace variant document the new auto behavior. Also reconciled the inplace docstring, which incorrectly claimed AITER consumes the cos/sin tables in the query dtype — the C++ shim (rope_aiter.cu) passes them as float32.
• tests/rocm_tests/test_activation_hip.py, test_rope_hip.py — added test_*_auto_backend_selection asserting auto resolves to "native".
• tests/rocm_tests/test_activation_aiter_hip.py, test_rope_aiter_hip.py — removed the old auto-selection tests from the AITER modules. Those modules are gated behind requires_aiter (skipped when the aiter package is absent), but the auto-default contract is aiter-independent, so the tests now live in the native modules where they run on any HIP build.
• README.md — Feature Support Matrix (RoPE, Activation rows) and the AITER Support prose updated to reflect auto→native for these two ops; the C++ AITER path remains opt-in.

Architecture / design notes

The four C++-level AITER ops now split on auto-routing:

Op	backend="auto" resolves to
rmsnorm	AITER (2-D fp16/bf16 on gfx942/gfx950), else native
fused_add_rmsnorm	AITER on gfx942/gfx950, else native
silu_and_mul	native (AITER opt-in via backend="aiter")
rope (cos/sin-cache)	native (AITER opt-in via backend="aiter")

The _auto_select_*_backend helpers are kept (rather than inlining "native") as the uniform extension point and test seam, matching the pattern in norm.py.

Test plan

• pytest tests/rocm_tests/test_activation_hip.py tests/rocm_tests/test_activation_aiter_hip.py tests/rocm_tests/test_rope_hip.py tests/rocm_tests/test_rope_aiter_hip.py -n auto --reruns 2 — all pass on gfx942 with aiter installed
• pre-commit run -a

[Copilot]

Pull request overview

This PR changes the HIP backend="auto" routing for silu_and_mul and cos/sin-cache RoPE ops to always select the in-tree native implementation (leaving AITER reachable only via backend="aiter"), and updates docs/tests/README to match that contract.

Changes:

• Make _auto_select_silu_and_mul_backend and _auto_select_rope_backend always return "native".
• Update public docstrings and README AITER/auto-routing documentation to reflect auto→native for these ops.
• Move/add tests so auto-backend-selection assertions run independent of the aiter Python package.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
flashinfer/activation.py	Forces auto backend selection for silu_and_mul to native and updates docstring.
flashinfer/rope.py	Forces auto backend selection for cached RoPE to native and updates docstrings.
tests/rocm_tests/test_activation_hip.py	Adds an auto-backend-selection test asserting auto→native.
tests/rocm_tests/test_activation_aiter_hip.py	Removes the old auto-selection test from the AITER-gated suite.
tests/rocm_tests/test_rope_hip.py	Adds an auto-backend-selection test asserting auto→native (including mixed dtype case).
tests/rocm_tests/test_rope_aiter_hip.py	Removes the old auto-selection test from the AITER-gated suite.
README.md	Updates the support matrix and AITER support section to document auto→native for these two ops.

Comments suppressed due to low confidence (1)

flashinfer/rope.py:1220

• The docs now state that backend supports only 'auto', 'native', and 'aiter', and that 'aiter' is ROCm-only. However, on non-HIP platforms the function currently performs no backend validation (because the checks are under if IS_HIP:), so backend='aiter' or any unknown string silently falls back to the native implementation. This is inconsistent with the documented/expected API behavior and with activation.silu_and_mul, which raises on unsupported backends.

    backend : str
        Kernel backend to use. ``"auto"`` (default) resolves to the native
        FlashInfer JIT kernel on all platforms.
        ``"native"`` uses the FlashInfer JIT kernel on all platforms.
        ``"aiter"`` uses AMD AITER's C++ ``rope_cached_positions_2c_fwd_impl``
        kernel — ROCm (gfx942/gfx950) only; requires the ``aiter`` package and
        query/key to share a dtype.

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