Weight tying option has a bug

[knshnb]

If weight_tying option is True, only one update layer is called regardless of n_update_layers

https://github.com/pfnet-research/chainer-chemistry/blob/master/chainer_chemistry/models/gin.py#L37

[corochann]

So we should change this line to self.n_update_layers ?
https://github.com/pfnet-research/chainer-chemistry/blob/master/chainer_chemistry/models/gin.py#L92