feat(megamoe): add fp4 group16 to group32 requant path

Add a utility and synthetic benchmark path for converting FP4 group16 tensors into group32 tensors that the existing SM100 block32 MegaMoE kernels can consume.

Document that this is a requantization path rather than a lossless metadata rewrite, so GLM-5.2 accuracy validation is still required before production use.

Tested: PYTHONPYCACHEPREFIX=/private/tmp/deepgemm_pycache python3 -m py_compile deep_gemm/utils/math.py tests/test_layout.py tests/test_mega_moe.py

Tested: git diff --check

Not-tested: CUDA build, SM100/B300 runtime, and GLM-5.2 accuracy validation are not available locally.

megamoe-research-reports/glm52_nvfp4_group16_notes.md

@@ -15,6 +15,9 @@ carried an implicit group32 assumption.
   scale transpose with a group16-aware 128-element tiling.
 - `tests/test_mega_moe.py` exposes `--weight-gran-k 16|32` so synthetic runs can
   reproduce GLM-style group16 inputs without loading model weights.
+- `requant_fp4_to_gran_k(...)` provides an explicit experiment path for
+  converting a group16 FP4 checkpoint tensor into a group32 tensor that the
+  existing block32 kernels can consume.
 - The fused SM100 MegaMoE compute API now performs an explicit capability check
   for `recipe=(1, 1, 16)` instead of failing earlier with
   `Unknown SF transformation`.
@@ -33,6 +36,19 @@ group16 correctly requires auditing at least:
 Until that kernel work is complete and validated on B300/SM100, group16 should
 be treated as layout-supported but fused-compute unsupported.
 
+## Requantization path
+
+If the SM100 MXF4 MMA path cannot consume group16 scales directly, the fallback
+engineering path is checkpoint conversion:
+
+1. dequantize group16 FP4 values with their original scales;
+2. requantize the restored values with group32 scales;
+3. run the existing `recipe=(1, 1, 32)` MegaMoE kernel.
+
+This is not a lossless metadata rewrite. It changes the quantized checkpoint and
+must go through GLM-5.2 accuracy validation before it can be used as a production
+answer.
+
 ## Validation target
 
 After kernel support is added, validate with:
@@ -41,4 +57,6 @@ After kernel support is added, validate with:
 - `tests/test_layout.py` on SM100 for `gran_k=16`;
 - `tests/test_mega_moe.py --weight-gran-k 16 --ncu-profile-only` for synthetic
   fused execution;
+- `tests/test_mega_moe.py --weight-gran-k 16 --requant-group16-to-group32` for
+  the checkpoint conversion path against existing block32 kernels;
 - SGLang GLM-5.2 NVFP4 real-weight layout build and 8-card e2e smoke.