Runs forward+backward+Adam ungraphed to isolate whether H100
grad_buf=0 is caused by graph capture or the cuBLAS operation itself.
NVIDIA docs confirm cublasLtMatmul inside graphs can silently fail
on Hopper when algorithm uses internal workspace allocation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>