Add 6 new CUDA kernel functions to attention_backward_kernel.cu for the
batched cuBLAS attention path: attn_sdp_fwd, attn_sdp_bwd (sigmoid-gated
feature attention), attn_layer_norm_fwd, attn_layer_norm_bwd (per-sample
LayerNorm with residual), attn_bias_add, and attn_bias_grad_reduce.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>