return value of backward in class Sum

[ifoyooo]

From my perspective, the shape of 'grad_output' should be broadcasted into a_shape.
Although it can still pass task3_1, I'm not sure of its accuracy in more complex situations.

            @staticmethod
            def backward(ctx, grad_output):
                a_shape, dim = ctx.saved_values
              
                if dim is None:
                    out = grad_output.zeros(a_shape)
                    out._tensor._storage[:] = grad_output[0]
                    return out
                else:
                # START Code Update
                    return grad_output #should be replaced by add_zip(grad_output,zeros(a_shape)) #
                # END Code Update

[ifoyooo]

I just realized the case 2 in function 'expand' is exactly to tackle the problem aroused by the inconsistence between origin shape and gradient shape which is caused by the return value.

        # Case 2: Backward is a smaller than self. Broadcast up.
        true_shape = TensorData.shape_broadcast(self.shape, other.shape)
        buf = self.zeros(true_shape)
        self.backend._id_map(other, out=buf)
        if self.shape == true_shape:
            return buf

I'm sorry to trouble you.

[srush]

Thanks. Yes, this is not well documented. Just kind of snuck it in the fix this issue.