verifySigmoid function
void
verifySigmoid()
Implementation
void verifySigmoid() {
final x = Tensor.fromList([1, 1], [0.0]);
final loss = x.sigmoid().sum();
loss.backward();
final grads = x.grad;
bool ok = closeEnough(grads[0], 0.25);
print(
"SIGMOID: ${ok ? '✅ PASS' : '❌ FAIL (Expected 0.25, got ${grads[0]})'}",
);
}