[Bug]: Non-avx512 CPUs cannot use tensor parallelism

[zhinianqin]

🐛 Describe the bug

init_shm_manager is not implemented when NOT AVX512_FOUND, which makes it impossible to use tensor parallel on non-avx512 CPUs

Cause #15934

vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-7B -tp=2 --distributed-executor-backend mp

On CPUs without avx512 support or AVX512_DISABLED

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[zhinianqin]

@bigPYJ1151