array_to_scalar: pytorch doesn't support array_to_scalar

[ethancaballero]

e.g. the sum in last line (111) of:
https://github.com/mila-iqia/myia/blob/master/examples/mlp.py#L107-L111

def cost(model, x, target):
    """Square difference loss."""
    y = model.apply(x)
    diff = target - y
    return sum(diff * diff)