'Gives the (sub/super)gradient of the atom w.r.t. each argument.
Matrix expressions are vectorized, so the gradient is a matrix.
Args:
values: A list of numeric values for the arguments.
Returns:
A list of SciPy CSC sparse matrices or None.'

def _grad(self, values):

    (U, s, V) = LA.svd(values[0])
    ds = np.zeros(len(s))
    ds[0] = 1
    D = U.dot(np.diag(ds)).dot(V)
    return [sp.csc_matrix(D.ravel(order='F')).T]
