NVFP4 Mega MoE Kernel — Mojo Rewrite

Rewrite of the DeepGEMM fp8_nvfp4_mega_moe kernel in Mojo.

Why Mojo?

• Python-like syntax, C-level performance
• Direct GPU programming without PTX inline asm
• Safer than CUDA C++ (ownership, borrowing)
• Better ergonomics for complex kernel development

Architecture

The kernel performs NVFP4 (E2M1 + UE4M3 block16 scales) matrix multiply for MoE (Mixture of Experts) with expert parallelism across NVLink.

Key operations:

1. Staging — quantize BF16 activation to FP4 (E2M1) with UE8M0 scales
2. TMA load — load packed FP4 weights and UE4M3 scales from global memory
3. UMMA — mxf4nvf4 matrix multiply with block scaling
4. Epilogue — quantize L1 output (BF16 → FP4 + UE4M3 scales for L2)
5. NVLink sync — cross-rank barrier and buffer management

NVFP4 specifics (vs MXFP4):

• group_size=16 (UE4M3 block scales), not group_size=32 (UE8M0)
• 2 SF K-columns per BLOCK_K (128/16/4=2), not 1
• Weights are E2M1 packed int8 (2 values per byte)
• mxf4nvf4 UMMA instruction with scale_vec::4X

Structure

src/
  mega_moe.mojo    — main kernel entry point
  staging.mojo     — activation quantization (BF16 → FP4)
  tma.mojo         — TMA descriptor creation and copy
  umma.mojo        — UMMA descriptor and MMA operations
  epilogue.mojo    — output quantization and TMA store
  barrier.mojo     — NVLink cluster sync and symm buffer
  layout.mojo      — weight transformation and SF layout
  utils.mojo       — math helpers, UE4M3 packing