Add instance key to training graph

1. The entire URL of the file you are using

https://github.com/tensorflow/models/tree/master/research/object_detection

2. Describe the feature you request

User can choose to add instance key to training graph.

3. Additional context

For batch predictions, user must pass instance keys unique to each image. To manage this case, the training graph should include a key that passes through the graph untouched (via tf.identity?). As I don't know how and where to add this "key" input, I would like a feature that does that automatically when specified by the user.

4. Are you willing to contribute it? (Yes or No)

Unfortunately no