DEBLEU example with TeacherMaskSoftmaxEmbeddingHelper and Triggers

[wwt17]

This example demonstrates the usage of Differentiable Expected BLEU (DEBLEU) with teacher mask and triggers. See https://openreview.net/pdf?id=S1x2aiRqFX for the implemented paper. The IWSLT'14 English-French dataset is to be added.