text: Title: t2t_decoder hangs when dot_product_relative_v2 is used
  Body: Hello,
  
  I am trying to train a custom Transformer model that has a decoder only (with a custom bottom['targets']), for sequence generation. I was able to train and generate from the model when I had not specified any other special params. However, the generated sequences frequently had a failure mode where certain tokens repeated too often. 
  
  I then added the two following params and am training a new model.
    hparams.self_attention_type = "dot_product_relative_v2"
    hparams.max_relative_position = 256
  
  However, now when I run t2t_decoder, it hangs and does not generate any output (and it's hard to kill it with ^C, and I have to do a kill -9).
  I run the decoder in interactive mode, and simply press the return at the '>' prompt. 
  
  t2t_decoder   --data_dir="${DATA_DIR}"   --decode_hparams="${DECODE_HPARAMS}"   --decode_interactive   --hparams="sampling_method=random"   --hparams_set=${HPARAMS_SET}   --model=${MODEL}   --problem=${PROBLEM}   --output_dir=${TRAIN_DIR}
  
  where:
  
  DECODE_HPARAMS="alpha=0,beam_size=1,extra_length=2048"
  MODEL=transformer
  
  OS: macOS, High Sierra
  
  $ pip freeze | grep tensor
  Error [Errno 20] Not a directory: '/Users/vida_vakil/miniconda3/lib/python3.6/site-packages/magenta-1.0.2-py3.6.egg' while executing command git rev-parse
  Exception: 
  ....
  NotADirectoryError: [Errno 20] Not a directory: '/Users/vida_vakil/miniconda3/lib/python3.6/site-packages/magenta-1.0.2-py3.6.egg'
  
  The model I am using is based on Score2Perf (https://github.com/tensorflow/magenta/tree/master/magenta/models/score2perf), and I have installed it using instructions from their page, and here: https://github.com/tensorflow/magenta
  Looks like the error has to do with the egg thing.
  
  $ python -V
  Python 3.6.6 :: Anaconda, Inc.
  
  tensorflow   1.12.0
  tensor2tensor   1.13.0
  
  Thanks in advance