Calculate gradient

def calc_grad(exe, exe_grads, params, X, Y, label_name=None, outgrad_f=None):
    """Calculate gradient"""
    exe.copy_params_from(params)
    exe.arg_dict['data'][:] = X
    if outgrad_f is None:
        exe.arg_dict[label_name][:] = Y
        exe.forward(is_train=True)
        exe.backward()
    else:
        exe.forward(is_train=True)
        exe.backward(outgrad_f(exe.outpus, Y))
    for k, v in exe_grads.items():
        v.wait_to_read()