Python 3 compatibility issue with `self_generated_targets`

[ywkim]

Description

I get AssertionError when trying to return self_generated_targets in a model.

Environment information

OS: macOS 10.13.4

$ pip freeze | grep tensor
mesh-tensorflow==0.0.4
tensor2tensor==1.11.0
tensorboard==1.12.0
tensorflow==1.12.0
tensorflow-metadata==0.9.0
tensorflow-probability==0.5.0

$ python -V
Python 3.6.4

For bugs: error logs

  File "/Users/ywkim/tensor2tensor/tensor2tensor/utils/t2t_model.py", line 1266, in wrapping_model_fn                                                                 
    use_tpu=use_tpu)
  File "/Users/ywkim/tensor2tensor/tensor2tensor/utils/t2t_model.py", line 1340, in estimator_model_fn                                                                
    "Found {}".format(logits.keys())
AssertionError: Expect only key 'logits' when there is 'self_generated_targets'. Found dict_keys(['logits'])