[Bugfix][Quantization] Reduce NVFP4 MoE load memory under sleep mode

[hubunt]

Purpose

Fixes #48680.

NVFP4 ModelOpt MoE conversion retained four lists of per-expert transformed tensors and then created four additional full-size outputs with torch.stack. Under --enable-sleep-mode, those overlapping allocations increase pressure on the CuMemAllocator private pool during weight loading.

This change preallocates the final W13/W2 weight and scale tensors, writes permuted weights directly with torch.index_select(..., out=...), and reuses one scale scratch buffer per scale type. It removes the per-expert result lists and final stack copies without changing the output layout.

This is not a duplicate of #45589: that change addressed the FlashInfer TRTLLM BF16 MoE conversion path, while this PR addresses the separate ModelOpt NVFP4 conversion path. It is intentionally a path-specific fix and does not change CuMemAllocator globally.

AI assistance (OpenAI Codex) was used; the submitter reviewed the resulting changes.

Test Plan

• Verify the optimized conversion is byte-for-byte identical to the reference row permutations.
• Ensure the conversion no longer calls torch.stack.
• Run focused unit tests and relevant formatting/lint checks.
• Ask the [Bug]: --enable-sleep-mode OOMs loading an NVFP4 (modelopt) 30B on 16GB SM120 cards where the identical model loads fine without it — cumem MemPool overhead, not resolved by raising max_split_size_mb #48680 reporter to validate the affected NVFP4 model with --enable-sleep-mode on SM120 hardware before marking this Draft ready.

Test Result

• .venv/bin/python -m pytest tests/kernels/moe/test_flashinfer_fp4_moe_utils.py -q: 1 passed.
• Ruff check and format checks passed.
• Python compilation and git diff --check passed.
• Applicable pre-commit hooks passed.
• Manual old/new algorithm comparison produced byte-for-byte identical outputs.
• A synthetic Nemotron-like conversion shape reduced measured transient peak allocation from 0.996 GiB to 0.793 GiB (20.4%).

End-to-end SM120 model validation is requested from the issue reporter before this Draft is marked ready.

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[hubunt]

@JasonVranek, could you please validate this Draft on the SM120 setup from #48680? In particular, please check that the NVFP4 model loads with --enable-sleep-mode using the original reproduction configuration, and that a basic inference request returns correct output. The branch is hubunt:fix/nvfp4-sleep-mode-load-oom. Once that validation is available, this PR can be marked ready and a reviewer can trigger the full CI run.