def reduce_func(nodes):
    """Collect messages and update node representations.

    Parameters
    ----------
    nodes : NodeBatch
        A batch of nodes.

    Returns
    -------
    dict mapping 'hv_new' to Float32 tensor of shape (V, K * T)
        Updated node representations. V for the number of nodes, K for the number of
        radial filters and T for the number of features to use
        (types of atomic number in the paper).
    """
    return {'hv_new': nodes.mailbox['m'].sum(1)}