Bug in losses_utils.reduce_dimensions

[gpengzhi]

https://github.com/asyml/texar/blob/master/texar/losses/losses_utils.py#L166
If average_axes and sum_axes are integers, len(average_axes) (https://github.com/asyml/texar/blob/master/texar/losses/losses_utils.py#L183) and len(sum_axes) (https://github.com/asyml/texar/blob/master/texar/losses/losses_utils.py#L190) will cause error since object of type 'int' has no len().