[Enhancement] Patch OmniStage.try_collect() with _proc alive checks

[pi314ever]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Waiting for OmniStage involves checking the output queue for results. However, try_collect() does not check if process has died and will hang indefinitely. This fixes this issue by explicitly checking that the process is alive before attempting to read the output queue. Component of #1557 relating to issue #1346

Test Plan

Test Result

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[pi314ever]

@xuechendi

[xuechendi]

LGTM

[pi314ever]

@hsliuustc0106 ^

[hsliuustc0106]

@ApsarasX PTAL

[lishunyang12]

Left a couple comments. The main one is a race between the is_alive() check and the queue read — the current ordering can discard valid results that are already queued when the worker exits normally.

[pi314ever]

@lishunyang12 I resolved your comments with minor tweaks.