Intialize io_thread_pool attribute in the beginning.

[rabi]

We check for io_thread_pool attribute in shutdown() before it could be initialized and results in AttributeError. Should be intialized to None in the beginning before we create a ThreadPoolExecutor.

FIX #18302

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[chaunceyjiang]

Thanks~

[rabi]

/retest

[rabi]

@comaniac @WoosukKwon can you have a look please? Looks like the issue was introduced in #14219

[comaniac]

LGTM. Thanks for catching