model.summary() and plot_model() doesn't show full structure of the model

[skshahidur]

With the wrapper classes ModelWithCRFLoss and ModelWithCRFLossDSCLoss the model.summary() and plot_model() method doesn't work properly. It shows only one single layer. So it's better to enable the graph property so that the structure is visible. This is common for any subclassed model in tf.

Ref: https://github.com/xuxingya/tf2crf/pull/44