[Bugfix] Add a aiter enabled check before appending a unimported class

[JadenMathias]

Purpose

A quick fix addressing a NameError that is caused when running serve command with aiter disabled:

MODEL=amd/Llama-3.3-70B-Instruct-FP8-KV
MAX_MODEL_LEN=10240
TP=1
CONC=32
INPUT_LEN=1024
OUTPUT_LEN=1024
ATTN_BACKEND="--attention-backend ROCM_ATTN"
FUSE_ROPE_KVCACHE="-cc.pass_config.fuse_rope_kvcache=True -cc.use_inductor_graph_partition=True"
FUSE_OTHER="-cc.pass_config.fuse_norm_quant=True -cc.pass_config.fuse_act_quant=True -cc.pass_config.fuse_attn_quant=True"
# export VLLM_ROCM_USE_AITER_TRITON_FUSED_ROPE_ZEROS_KV_CACHE=1

export AMDGCN_USE_BUFFER_OPS=1
export VLLM_ROCM_USE_AITER=0 VLLM_ROCM_USE_AITER_MHA=0
export VLLM_ROCM_QUICK_REDUCE_QUANTIZATION=INT4
export VLLM_DISABLE_COMPILE_CACHE=1
# export VLLM_COMPILE_CACHE_SAVE_FORMAT=unpacked

rm -rf ~/.cache/vllm /tmp/torchinductor*
vllm serve $MODEL \
    --tensor-parallel-size=$TP --gpu-memory-utilization=0.94 \
    --dtype=auto --kv-cache-dtype=fp8 \
    --max-num-batched-tokens=131072 \
    $ATTN_BACKEND \
    $FUSE_ROPE_KVCACHE \
    $FUSE_OTHER \
    --compilation-config '{"use_inductor_graph_partition": true}' \
    --no-enable-prefix-caching \
    --disable-log-requests \
    --disable-uvicorn-access-log \

The following error is observed:

This fix simply adds the check for if aiter is enabled

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