parse head gradient for backward and grad.

def _parse_head(heads, head_grads):
    """parse head gradient for backward and grad."""
    if isinstance(heads, NDArray):
        heads = [heads]
    if isinstance(head_grads, NDArray):
        head_grads = [head_grads]

    head_handles = c_handle_array(heads)

    if head_grads is None:
        hgrad_handles = ctypes.c_void_p(0)
    else:
        assert len(heads) == len(head_grads), \
            "heads and head_grads must be lists of the same length"
        hgrad_handles = c_array(NDArrayHandle,
                                [i.handle if i is not None else NDArrayHandle(0)
                                 for i in head_grads])
    return head_handles, hgrad_handles