[ROCm] Add env to enable/disable aiter triton gemm

[sarckk]

Purpose

#26969 added usage of AITER gemm_a16w16 for GEMM ops on AMD for valid shapes. However, it passes a bias term to gemm_a16w16 that was only added to AITER in ROCm/aiter#1148 (merged Oct 28).

Some are still using an older version of AITER. For backward compatibility, we should have an option to disable using this AITER triton gemm kernels.

Test Plan

VLLM_ROCM_USE_AITER_TRITON_GEMM=0 VLLM_ROCM_USE_AITER=1  vllm serve Qwen/Qwen3-8B

Test Result

Checked that VLLM_ROCM_USE_AITER_TRITON_GEMM=0 disables it and by default on.

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[gemini-code-assist]

Code Review

This pull request introduces an environment variable VLLM_ROCM_USE_AITER_TRITON_GEMM to control the usage of AITER's Triton GEMM kernel, which is a good addition for backward compatibility. The implementation is straightforward. However, there is a critical issue: the new environment variable, which affects the computation graph, has not been added to the compute_hash function. This can lead to incorrect caching behavior. I've added a comment with details on how to fix this.

[sarckk]

cc: @gshtras @maleksan85

[tjtanaa]

@sarckk May I know if you are evaluating against the ROCm aiter version that is specified in the Dockerfile.rocm_base? The aiter code that are merged into main are supposed to be based off that version. I will double check on my end if the gemm_a16w16 is suppored in the ROCm aiter defined in Dockerfile.rocm_base.

[tjtanaa]

@sarckk It seems that the API is valid for Dockerfile.rocm_base. Thank you for surfacing there are situation where we need to support backwards compatibility. We will find a way to go forward.
We will try to avoid introduce new flags as it is causing confusion for users as there are too many flags to tuned.

[sarckk]

@tjtanaa yea in general it would be good to have some option to easily toggle features that rely on the latest AITER version, thanks