Module backward 

Backward activation and utility shaders for backpropagation.

All shaders use workgroup size 256 (1D) and follow the same dispatch pattern as the elementwise forward shaders.

Constants

REDUCE_COLS

Column-wise reduction: out[j] = sum_i(in[i * cols + j]) * scale.

RELU_BACKWARD

ReLU backward: out[i] = grad[i] * (z[i] > 0 ? 1 : 0).

SCALE

Element-wise scale: out[i] = in[i] * alpha.

SIGMOID_BACKWARD

Sigmoid backward: out[i] = grad[i] * a[i] * (1 - a[i]).

TANH_BACKWARD

Tanh backward: out[i] = grad[i] * (1 - a[i]^2).

TRANSPOSE

Matrix transpose: out[col * rows + row] = in[row * cols + col].