Fix: remove_sub_pool should kill subprocess gracefully

[frostyplanet]

This fix bug of xinference:

Only the main process got kill, but subprocess leaves behind

before terminate-model: root 179070 1942 1.3 60529056 7144300 ? Rl 04:22 9:23 _ Model: qwen2-instruct:72-0-1722111769.2038095 root 179297 2395 1.3 59784688 6867800 ? Rl 04:22 9:34 | _ Model: qwen2-instruct:72-0-1722111769.2038095 root 179300 2373 1.3 59784740 6881352 ? Rl 04:22 9:29 | _ Model: qwen2-instruct:72-0-1722111769.2038095 root 179301 2362 1.2 59747064 6858788 ? Rl 04:22 9:26 | _ Model: qwen2-instruct:72-0-1722111769.2038095

after terminate-model: root 179297 2395 1.3 59784688 6867800 ? Rl 04:22 9:34 _ Model: qwen2-instruct:72-0-1722111769.2038095 root 179300 2373 1.3 59784740 6881352 ? Rl 04:22 9:29 _ Model: qwen2-instruct:72-0-1722111769.2038095 root 179301 2362 1.2 59747064 6858788 ? Rl 04:22 9:26 _ Model: qwen2-instruct:72-0-1722111769.2038095

What do these changes do?

Related issue number

Fixes #xxxx

Check code requirements

• [ ] tests added / passed (if needed)
• [ ] Ensure all linting tests pass