verifySum function
void
verifySum()
Implementation
void verifySum() {
final x = Tensor.fromList([2, 2], [1.0, 2.0, 3.0, 4.0]);
final loss = x.sum();
loss.backward();
final grads = x.grad; // Assuming .grad also calls fetchData() internally
bool ok = grads.every((g) => closeEnough(g, 1.0));
print("SUM: ${ok ? '✅ PASS' : '❌ FAIL (Expected grads of 1.0, got $grads)'}");
}