feat: NVFP4 mega MoE kernel (scale_vec::4X, UE4M3 block scales)
- New CUDA kernel: sm100_fp8_nvfp4_mega_moe_impl
- kGranK=16 (NVFP4 group_size=16, vs MXFP4's 32)
- kind::mxf4nvf4.block_scale.scale_vec::4X PTX instruction
- float_ue4m3_t scale factor type in instruction descriptor
- SF layout: scale_vec::4X (4 TMEM sub-columns per UMMA atom)
- UTCCP column stride: i*8 (vs MXFP4's i*4) for 4X layout
- L1 epilogue: UE4M3 activation scales (float→cutlass::float_e4m3_t)
- SF loading: kNumSFUint32 = kHidden/64 (4 UE4M3 per int32)
- New PTX wrappers: SM100_MMA_MXF4NVF4_2x1SM_SS, SM100_MMA_MXF4NVF4_SS
- Python API:
- fp8_nvfp4_mega_moe() with recipe=(1,1,16)
- transform_nvfp4_weights_for_mega_moe() for UE4M3→int32 UTCCP packing
- _pack_nvfp4_sf_for_utccp() helper
- C++ bindings:
- mega_nvfp4.hpp with NVFP4-specific SymmBuffer (SF stride K/16)
- JIT kernel header with kGranK=16 TMA descriptors
- Registered in python_api.cpp
NOTE: Both SFA and SFB must use UE4M3 (scale_format_ is 1-bit, shared).
The L1 epilogue converts float→UE4M3 for activation scales.
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