[Bugfix] Pad misaligned MXFP4 MoE shards for Marlin

[dumko2001]

Purpose

Marlin repacking fails when an expert-parallel MXFP4 shard has a local
intermediate size that is not aligned to Marlin's 64-element tile. Pad the
gate/up weights and scales, plus the packed down weights and scales, before
repacking. The padded values use zero scales and do not change the logical
expert output.

Fixes #47769

Related work: I searched open and closed PR references for #47769 and found no
overlapping PR. AI assistance was used; I reviewed the changed code and the
GPU evidence.

Test Plan

uv run --no-project python repro/repro_marlin_mxfp4_alignment.py
uv tool run ruff check \
  repro/repro_marlin_mxfp4_alignment.py \
  repro/gpu_validate_mxfp4_marlin_alignment.py
uv run --no-project python -m py_compile \
  vllm/model_executor/layers/quantization/utils/marlin_utils_fp4.py \
  repro/repro_marlin_mxfp4_alignment.py \
  repro/gpu_validate_mxfp4_marlin_alignment.py

On H100, the GPU script ran with e=2, n=130, and k=256, first with the
stock wheel and then after overlaying:

vllm/model_executor/layers/quantization/utils/marlin_utils_fp4.py

Added CPU shape coverage for the misaligned shard. A follow-up commit also
corrected padding for the packed down-weight and scale layouts.

Test Result

The unpatched run reproduced:

RuntimeError: gptq_marlin_repack ... size_n = 130 is not divisible by tile_n_size = 64

The patched run succeeded and printed the Marlin tile-padding warning. It
returned:

w13       (2, 16, 768)
w2        (2, 12, 512)
w13_scale (2, 8, 384)
w2_scale  (2, 6, 256)

The GPU script now asserts these post-repack shapes. A fused-MoE numerical
comparison against a dequantized reference remains future work.

---

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