CaptioningExample miss get_example_id function

[Flaick]

Hello, when I run the training with mscoco caption, I found the error during the evaluation, because of the missing get_example_id in CaptioningExample under data/coco_captioning.py

    def get_example_id(self):              
      return self.example_id          

I update the code in this way, now it works for me, is that correct?