[ROCm][AITER] bugfix accuracy regression in ROCM_AITER_TRITON_MLA backend

[vllmellm]

Purpose

Running deepseek using the AITER_TRITON_MLA backend results in low accuracy as follow:
command:
export VLLM_USE_V1=1 export SAFETENSORS_FAST_GPU=1 export VLLM_ROCM_USE_AITER=1 export VLLM_ATTENTION_BACKEND=ROCM_AITER_TRITON_MLA vllm serve deepseek-ai/DeepSeek-V3 -tp 8
lm_eval score:

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.0067	±	0.0047
		strict-match	5	exact_match	↑	0.0000	±	0.0000

This PR fixes the issue by inheriting from AiterMLABackend instead of MLACommonBackend since AiterMLABackend in shared for aiter triton backend as well.

Test Plan

verify lm_eval score

export VLLM_USE_V1=1 export SAFETENSORS_FAST_GPU=1 export VLLM_ROCM_USE_AITER=1 export VLLM_ATTENTION_BACKEND=ROCM_AITER_TRITON_MLA vllm serve deepseek-ai/DeepSeek-V3 -tp 8

lm_eval --model local-completions \ --tasks gsm8k \ --model_args model=deepseek-ai/DeepSeek-V3,base_url=http://localhost:8000/v1/completions \ --trust_remote_code \ --num_fewshot 5 \ --limit 300 \ --batch_size 128

Test Result

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.96	±	0.0113
		strict-match	5	exact_match	↑	0.96	±	0.0113

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[gemini-code-assist]

Code Review

This pull request addresses a critical accuracy regression in the ROCM_AITER_TRITON_MLA attention backend. The fix involves changing the base class of AiterTritonMLABackend from MLACommonBackend to AiterMLABackend. This change correctly allows AiterTritonMLABackend to inherit necessary configurations, like get_supported_kernel_block_sizes, from the shared AiterMLABackend, which is the intended base for AITer backends on ROCm. The changes also include cleaning up imports and removing a redundant method override, improving code clarity. The fix is logical, well-contained, and its effectiveness is demonstrated by the significant improvement in the provided lm_eval scores.

[tjtanaa]

LGTM. Thank you for fixing it.