[Bugfix] Fix DeepSeek V2-Lite Accuracy drop

[bnellnm]

Purpose

Fix DeepSeek V2-Lite Accuracy drop introduced by #40560

Test Plan

Run bash .buildkite/scripts/scheduled_integration_test/deepseek_v2_lite_ep_eplb.sh 0.25 200 8010
Ran + verified all models from #39956
CI MoE Refactor tests

Test Result

Accuracy went from 0.02 -> 0.35

cc @robertgshaw2-redhat

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[gemini-code-assist]

Code Review

This pull request replaces the _fused_output_is_reduced property with an instance attribute initialized in __init__ and _replace_quant_method. A critical issue was identified regarding the timing of this initialization: since the moe_kernel is typically lazily initialized during the first forward pass, caching the value in __init__ will likely result in a stale False value. This could lead to correctness issues, such as the double reduction of fused outputs, because the attribute will not reflect the kernel's state once it is actually loaded.

[robertgshaw2-redhat]

We should move the SP reductiuon into the runner soon

[mergify]

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