The exposure aux backward GEMM amplifies gradients through the
hidden activation matrix (d_logits × h_b0), producing grad_norm=5000+.
With max_grad_norm=10, this throttles ALL gradients by 500x (lr/500).
Fix: reduce aux_weight 0.5→0.01 (50x) + clip_grad_buf_inplace after
aux GEMM to cap combined gradient at max_grad_norm.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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