sum

import grain.testing;
import numir;
import mir.ndslice;
import std.meta;

auto hx = [1f, 2f, 3f, 4f].sliced(2, 2).variable(true);
auto loss = sum(hx);
assert(loss.data[0] == 10f);
loss.backward();
assert(hx.gradSlice == [1f, 1f, 1f, 1f].sliced(2, 2));