[DLight] Fix GPU reduction ScheduleError on 4D non-last-axis reductions

[cchung100m]

Hi Committers,

This PR fixes issue #19517. Any suggestions would be appreciated if you are available.

[gemini-code-assist]

Code Review

This pull request improves the robustness of the GPU reduction scheduling in dlight by dynamically identifying spatial and reduction loops based on their iteration types rather than assuming a fixed structure. It also introduces validation checks to safely return None if the loop structure does not match expectations. The review feedback points out a minor parenthesis typo in a newly added comment block.

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